I Stloukal

J Ermák

Silver diffusion in commercial QE22 magnesium alloy with Saffil fiber reinforcement

Keywords: A. Metal-matrix composites (MMCs), A. Short-fiber composites, A. Magnesium, B. Interface, B. High-temperature properties, Silver diffusion

Diffusion of 110m Ag in commercial QE22 magnesium alloy with and without short Saffil fibers reinforcement is measured by the radiotracer technique in the temperature interval 648 -728 K. The obtained diffusion coefficients D v in the experimental alloys without reinforcement allow to evaluate the influence of the interface between matrix and Saffil fibers on the effective diffusion coefficient D eff and estimate the interface diffusion coefficient D i of Ag in interphase boundaries. The results obtained for unreinforced alloy show typical Arrhenius behavior. The pre-exponential factor D 0 = ( ) -1 2 3 5 3 s m 10 6 -+ -× and activation enthalpy Q = 154 ± 3 kJ mol -1 are evaluated. It is observed that the temperature dependence of diffusion coefficients D eff and D i is not Arrhenius-like in the studied temperature interval. This behavior is attributed to relaxation of thermo-elastic stresses in the composite as an effect of large difference between coefficients of thermal expansion (CTE) of Saffil fibers and metal matrix. The maximum values of D eff and D i , respectively, lies close to 693 K, where CTE has a minimum.

Introduction

Magnesium-based alloys represent a large group of materials. They are produced in an extent of about 300 000 t/year. They are applied as structural applications, e.g. in military aircraft or automotive industry. They have many excellent physical properties:

Mg is the lightest metal, it has high specific strength, good cast ability, etc. [START_REF] Avedesian | Magnesium and Magnesium Alloys[END_REF]. On the other side, there are several properties that limit magnesium for application in industry: low elastic modulus, limited cold workability, limited strength and creep resistance at elevated temperatures, etc. To overcome these unsuitable properties, magnesium is alloyed by other elements and/or some reinforcement is used. Aluminum, zinc, manganese, silver, zirconium and the rare earth metals (RE) are the most frequently used alloying elements; and Al 2 O 3 and SiC are used as reinforcing materials.

The commercial creep resistant QE22 alloy is one example of rare earths containing magnesium alloys. Typical compositions of this alloy is Mg-Ag-Di-Zr, where Di is socalled didymium -a mixture of RE elements mainly Nd and Pr. Silver alloying improves mechanical properties of magnesium alloys, RE metals increase strength at elevated temperatures and zirconium is very effective grain refiner. Unfortunately, alloying does not improve the elastic properties, such as the Young's modulus.

Therefore, the ceramic fibers are suitable to improve stiffness and wear resistance of such metal matrix composites (MMC). The role of the reinforcement on mechanical properties QE22/Al 2 O 3 /44s MMC including effects on the interface matrix/fibers was intensively studied in recent years, see e.g. in [START_REF] Luká | Thermal expansion in magnesium composites[END_REF][START_REF] Rudajevová | Thermal expansion behaviour of Mg-Saffil fibre composites[END_REF][START_REF] Kumar | Analysing hysteresis and residual strains in thermal cycling curves of short fibre reinforced Mg-MMCs[END_REF][START_REF] Rudajevová | Thermal expansion of pre-deformed, short fibre reinforced Mg-based composites[END_REF][START_REF] Rudajevová | Influence of the interface on the thermal expansion and thermal conductivity of QE22 based composites[END_REF][START_REF] Mielczarek | Influence of mechanical cycling on damping behaviour of short fibre-reinforced magnesium alloy QE22[END_REF].

The QE22 alloy is used e.g. as structural material of landing wheels or as rotor heads for helicopters. In these systems, the components are under thermal and mechanical stress. Therefore, knowledge of diffusion characteristics is very important for evaluation of the rate of such diffusion controlled processes. In the case of MMC, the presence of ceramic fibers creates the matrix/fiber interfaces that may serve as the high diffusivity short-circuit paths. Finally, the considerable difference in the coefficient of the thermal expansion (CTE) of fibers and matrix causes internal stresses during temperature changes and subsequently some stress relaxation phenomena may occur.

The present work deals with measurements of silver diffusion in QE22 magnesium alloy with and without Saffil fiber reinforcement. The experiments were carried out with and without reinforced variant of QE22 alloys in the temperature interval 648-728 K using serial sectioning method. The obtained 110m Ag diffusion coefficients were compared with data on 65 Zn diffusion in the same materials in our previous work [START_REF] Stloukal | Diffusion of 65 Zn in AZ91 and in QE22 reinforced by Saffil fibers[END_REF].

The influence of CTE on the temperature dependence of the effective diffusion coefficient D eff and the interface diffusion coefficient D i will be discussed.

Experimental procedure

Sample preparation

The experimental QE22 magnesium alloy (QE22 is standard ASTM notation [START_REF] Avedesian | Magnesium and Magnesium Alloys[END_REF]) was prepared at the Technische Universität Clausthal-Zellerfeld, Germany by squeeze casting method. The chemical composition of investigated QE22 alloy was Mg -2.5 wt.% Ag -2.1 wt.% Di -0.7 wt.% Zr. The fiber preform consisted of planar randomly distributed Saffil short fibers. The standard composition of them is 97% δ-Al 2 O 3 + 3% SiO 2 , the mean fiber length is about 50 µm and diameter about 4 µm. The total volume fraction of Saffil (44 vol.%) was checked by digital image analysis of SEM micrographs.

Samples of QE22 without Saffil fibers (referred to as QE22 hereafter) were machined from marginal part of the ingot, where there was no reinforcement. Core part of the ingot was used for QE22 samples with Saffil fibers (QE22S). The cylinder samples were machined in such a way that the specimen axes were parallel to the plane in which the longitudinal axes of fibers was preferentially situated. The diameter of diffusion specimens was 10 mm and height 4 mm.

The standard T4 heat treatment (798 K / 8 h, water quenching) was used [START_REF] Avedesian | Magnesium and Magnesium Alloys[END_REF]. The samples were wrapped in Ta foils, encapsulated in silica ampoules filled with pure Ar and solution treated as mentioned above in horizontal tube furnace. The temperature was checked by a Pt-PtRh thermocouple with an accuracy of ± 1 K. The subsequent annealing of pre-heated samples was applied to stabilize the structure and grain size of alloys. The respective temperature T and time t were chosen in order to simulate diffusion anneals (Table 1). All temperatures were within the interval, where the only homogenous solid solution of Ag in Mg exists [START_REF] Massalski | Binary alloy phase diagram[END_REF].

The final structure of the experimental alloys is show in Figs. 1,2. The microstructure of QE22 alloy (Fig. 1.) consists of relatively large grains with mean size about 150 µm. The precipitation of Nd-rich phase (bright particles in Fig. 1.) was observed, which is segregated mostly at grain boundaries. Since the QE22 alloy contains more than 2 wt.% Ag, the formation of an equilibrium phase with Mg 12 Nd 2 Ag composition is expected as observed in [START_REF] Polmear | Magnesium alloys and applications[END_REF][START_REF] Rakowska | Analytical electron microscopy of magnesium alloy containing neodymium[END_REF]. However, no Ag was detected in those particles by SEM-EDX analysis. Therefore, the presence of such particles can be excluded. All observed precipitates had composition given by a formula Mg 9-12 Di and Ag is solved in the matrix. The same Nd-rich particles were observed in QE22S alloy, but they were preferentially located at the surface of fibers (Fig. 2.), as seen Fig. 3. In the case of QE22S, Saffil fibers might react with the melt during squeeze casting of the samples [START_REF] Rehman | Fibre/matrix interactions in magnesium-based composites containing alumina fibres[END_REF]. However, due to the short melt-fiber contact no interface reactions were observed. Although the precipitation of (Al x Mg 1-x )Nd phase in similar material was observed by Kiehn et al. [START_REF] Kiehn | Microstructure changes in isochronally annealed alumina fibre reinforced Mg-Ag-Nd-Zr alloy[END_REF], no Al content was detected by SEM-EDX analysis in the matrix of the studied QE22S. Therefore, the presence of Al-rich particles can be also excluded in reinforced alloy.

Radiotracer measurements

The pre-heated samples were metallographically ground and polished using standard methods to achieve a mirror-like quality of the surface. The OP-S suspension (colloidal silica in water, particles mean grain size 0.04 µm) was applied for the last polishing step. The radiotracer 110m Ag was used in the form of an aqueous solution of 110m AgNO 3 in 1M HNO 3 . The specific activity was 57 MBq mg -1 of Ag at the beginning of the experiments. The radiotracer was deposited on the sample surface using the vacuum evaporation technique in the following way: first, the carrier was dripped-and-dried onto a tungsten boat, then it was thermally decomposed in vacuum (~ 3 × 10 -6 mbar) and finally, it was evaporated onto cold samples. After that, the samples was wrapped in Ta foil, encapsulated in silica ampoules filled with pure Ar and annealed at selected temperatures T and diffusion times t (Table 1) in a horizontal furnace stabilized within ±

Results and discussion

Penetration profiles

The measured penetration profiles are shown in Figs. 4, 5 in co-ordinates log c vs. The values of mean diffusion path in the bulk, L, are listed in Table 1. With respect to microstructure of QE22S alloy in Fig. 2., it is clear that values of L are much greater than the typical distance between the fibers, and, at the same time, much greater than the typical diameter of fibers. Hence, it can be concluded that the diffusion in the reinforced alloy can be well described by an effective diffusion coefficient D eff [START_REF] Kaur | Fundamentals of grain and interphase boundary diffusion[END_REF].

Effective diffusivity

As can be seen in Table 1, the values of D eff are greater than those of D v , except at 728 K. If we suppose that volume diffusion in alumina in general is much slower [START_REF] Gall | Self-diffusion in α-Al 2 O 3 I. Aluminium diffusion in single crystals[END_REF] than in QE22, it may be concluded that the difference between D eff and D v is due to the fast diffusion along interface Saffil/matrix. Belova and Murch [START_REF] Belova | The effective diffusivity in polycrystalline material in the presence of interphase boundaries[END_REF] have proposed the following relation between the effective diffusivity D eff in two phases and interphase boundary diffusivity (

D i 2 2 i 2 2 i 2 1 1 i 1 1 i 1 2 2 i 2 2 i 2 1 1 i 1 1 i 1 2 2 1 1 i i eff
) 2
The indices 1 and 2 denote the individual phases; g and s are volume fractions and segregation factors of respective phases. If we assume in the present case that silver diffusivity in the interfaces is much faster than that in alumina and in the matrix, i.e. , and that silver does not segregate too much to the near-surface area both in the matrix and fibers, i.e. s 1 , s 2 ~ 1, the Eq. 2 can be simplified to eff i

2 1 i 2 2 D g g g D + + = . ( 3
)
Then, for the known volume fraction of fibers g 1 ∼ 0.44 in QE22S, g i can be estimated using the following values: typical diameter of fibers was about 4 µm and length 50 µm, and thickness of interface δ ~ 5 × 10 -10 m (commonly accepted grain boundary thickness [START_REF] Kaur | Fundamentals of grain and interphase boundary diffusion[END_REF]). Thus the final relation can be written as

eff 3 i 10 7 D D × ≅ . ( 4
)
Calculated values of D i are summarized in Table 1. Their typical relative error was about 63%.

Temperature dependence of D v

The temperature dependence of the obtained volume diffusion coefficients D v in QE22 alloy are shown in Fig. 6. It can bee seen that the experimental points can be described by the Arrhenius relation -straight line fitted to full points in Fig. 6. The diffusion characteristic, i.e. pre-exponential factor D 0 and activation enthalpy Q, are listed in Table 2 together with literature data.

As it can be seen in Fig. 6, the obtained diffusion coefficients of Ag in QE22 alloy are lower than those for Zn diffusion in the same material [START_REF] Stloukal | Diffusion of 65 Zn in AZ91 and in QE22 reinforced by Saffil fibers[END_REF]. This result can be explained by the relation between atomic radii of elements [START_REF] Slater | Atomic radii in crystals[END_REF] (r Ag = 0.16 nm and r Zn = 0.135 nm, respectively) -the greater radius, the slower diffusivity of respective elements. The similar ratio of known diffusivity of the same elements can be seen for polycrystalline Mg (see Table 2). However in this case, the effect of atomic radii is not so expressive as for Ag and Zn diffusion in QE22 alloy. The explanation of such an effect can be explained using the temperature dependence of local strain close to the matrix/fiber interfaces. Rudajevová and Padalka [START_REF] Rudajevová | Thermal expansion of pre-deformed, short fibre reinforced Mg-based composites[END_REF] have studied thermal expansion of magnesium composites reinforced by Saffil fibers. It was observed that above the certain inverse temperature T i , the CTE of composite starts to decrease with increasing of temperature and it goes through a local minimum T CTE min .

There are three concurrent processes running parallel. (i) The volume diffusion described by coefficient D v , which increases with increasing temperature due to the thermal activation. This process is described by the Arrhenius equation. (ii) Increasing strain localized by interphase plane (matrix/fiber), which also increases with increasing temperature due to considerably different CTE. This leads, after the limits of elastic accommodation is exhausted, to an increase of dislocation density. (iii) With increasing the temperature, the recovery of dislocations also proceeds. This is more pronounced at higher Therefore, it can be supposed that all these effects superimpose one over another resulting in the increasing dislocation density up to the temperature about 688 K and decreasing dislocation density at higher temperatures. This leads to the observed break on the temperature dependence of D eff a D i as can be seen in Fig. 6.

Conclusions

The investigation of 110 Ag diffusion in magnesium-based QE22 alloy with and without Saffil fiber reinforcement yield the following conclusions: reported in our previous work [START_REF] Stloukal | Diffusion of 65 Zn in AZ91 and in QE22 reinforced by Saffil fibers[END_REF].

5. Similar to 65 Zn diffusion coefficients, the up-ward curvature of 110m Ag coefficients D eff and D i was observed close to T CTE min . This observation is probably caused by increasing dislocation density up to T ∼ 688 K and its decrease at higher T as result of recovery.

Table 1 Volume diffusion coefficients D v in QE22 alloy, effective diffusion coefficients D eff and diffusions coefficients D i in interfaces between the matrix and Saffil fibers in QE22S alloy.

T [K] t [s] D v [m 2 s -1 ] t D L v 2 = [µm] D eff [m 2 s -1 ] D i [m 2 s -1 ]
728 7200 6 × 10 of the annealing different temperatures (Table 1).

Fig. 5. Penetration profiles for 110m Ag diffusion in QE22S alloy measured after variations of the annealing period at different temperatures (Table 1).

  diffusion treatment, the cylindrical samples were reduced in diameter to eliminate any possible effects of surface diffusion. The penetration profiles were obtained by the serial sectioning method along the samples' faces. The thin layers of about 2 -5 µm were removed from the sample surface by the Fully Motorized Rotary Microtome Leica RM2255. Each layer was carefully placed into a plastic vial and solved with 0.6 cm 3 of HCl solution (2 cm 3 conc. HCl + 100 cm 3 distilled water). After dissolution of the layers, each vial was filled with scintillation cocktails Ultima Gold. The isotope 110m Ag has a half-life 249.85 days and decays by emission of electrons with a maximum energy level 529 keV into stable isotope 110 Ag. The activity of each dissolved layer c(x,t), where x is penetration depth, was measured with a great efficiency using Liquid Scintillation Counter TRI-CARB 3170 TR/LS equipped with an automatic sample changer.

x 2 .

 2 As can be seen, there is good linearity of profiles over about 2 decades. This fact gives possibility to describe diffusion by volume diffusion coefficient D v in nonreinforced QE22 alloy and by effective diffusion coefficient D eff in reinforced one[START_REF] Kaur | Fundamentals of grain and interphase boundary diffusion[END_REF].The diffusion coefficients D v and D eff were obtained by fitting thin-source solution[START_REF] Crank | Mathematics of diffusion[END_REF] where M stands for surface concentration of diffusant at t = 0, D is either volume diffusion coefficient D v or effective diffusion coefficient D eff . Eq. (1) was fitted to measured penetration profiles and constants D and M were found as fit parameters. The obtained diffusion coefficients D are listed in Table1. Typical relative error was about 5% by D v and 20% by D eff . The near-surface part of the penetration profiles was excluded from the fitting procedure because of imperfection of first several cuts.

3. 4 .

 4 Temperature dependence of D eff and D i In comparison with D v , the temperature dependences of Ag diffusion coefficients D eff and D i show obviously a non-Arrhenius behavior in the measured temperature interval 648 -728 K. The deviation from the standard Arrhenius behavior is illustrated in Fig. 6 by dashed lines (they guide the eye only). The values of both diffusion coefficients D eff and D i , respectively, have a tendency to higher values in the middle of measured temperature interval. This behavior supports our previous observations dealing with zinc diffusion in QE22S alloy [8].

1 . 4 .

 14 The diffusion experiments were carried out in the temperature interval 648 -728 K. The obtained penetration profiles were used to calculate volume diffusion coefficients D v in the case of QE22 alloy and effective diffusion coefficients D eff in the case of QE22S alloy. 2. The observed relation D eff > D v invokes that the effective diffusivity in QE22S alloy consists of two contributions: the volume diffusion and the fast diffusion along the interface short Saffil fibers/matrix. 3. The interphase boundary diffusivity D i was estimated using the theoretical relation between D eff and D i , known geometrical properties of the Saffil fibers and the commonly used value of width of the interface boundary. The relation between D eff and D i can be written as The temperature dependence of volume diffusion has Arrhenius behavior. The relevant parameters are D 0 = 6 × 10 -3 m 2 s -1 and Q = 154 kJ mol -1 . The silver volume diffusion coefficients are lower than analogous values of 65 Zn diffusion

Fig. 1 .

 1 Fig. 1. SEM micrograph of QE22 after T4 + 728 K / 2h treatments. Polished with

Fig. 2 .

 2 Fig. 2. SEM micrograph of QE22S after T4 + 728 K / 2h treatments. Polished with OP-

Fig. 3 .

 3 Fig. 3. SEM micrograph of QE22S after T4 + 728 K / 2h treatments. Polished with OP-

Fig. 4 .

 4 Fig. 4. Penetration profiles for 110m Ag diffusion in QE22 alloy measured after variations

Fig. 6 .Fig. 1 .Fig. 2 .

 612 Fig. 6. Arrhenius plot of measured 110m Ag diffusion coefficients in QE22 and in the fiber reinforced alloy -full symbols compared with data on 65 Zn diffusion in the same

Fig. 3 .Fig. 5 .Fig. 6 .

 356 Fig. 3.

Table 2

 2 Arrhenius parameters for 110 Ag diffusion in QE22 alloy in comparison with 110 Ag solute diffusion in pure Mg single crystal and with 65 Zn diffusion in QE22 alloy.

	Material	Diffusant D 0 [m 2 s -1 ] Q [kJ mol -1 ] Reference
	QE22	110m Ag	( ) 3 5 3 10 6 -+ -×	154 ± 3	this work
	QE22	65 Zn	9 × 10 -5	125	[8]
	single crystal Mg (|| to c-axis) 110m Ag	4 × 10 -4	133	[18]
	single crystal Mg (⊥to c-axis) 110m Ag	2 × 10 -3	148	[18]
	polycrystalline Mg	65 Zn	4 × 10 -5	120	[19]
	polycrystalline Mg	110m Ag	3 × 10 -5	119	[19]

Acknowledgements

The work was supported by the Czech Science Foundation (project No. 106/05/2115) and by the Academy of the Sciences of the Czech Republic (project No. AV0Z20410507).


Ulrich Ko
email: ulrich.koeppen@lung.mv-regierung.de

Alexander Petrovich Yakovlev

Raimund Barth
email: barth@orn.mpg.de

Michael Kaatz
email: michael.kaatz@storchenhof-loburg.de

Peter Berthold
email: berthold@orn.mpg.de

Seasonal migrations of four individual bar-headed geese Anser indicus from Kyrgyzstan followed by satellite telemetry

Keywords: Bar-headed goose, Anser indicus, Kyrgyzstan, Satellite telemetry, Migration routes, Stopover areas, Wintering areas

The Kyrgyz population of the bar-headed goose Anser indicus has declined dramatically during the past decades. Human persecution during migration and habitat loss at stopover and wintering sites are commonly regarded as most serious threats. However, little is known about seasonal movements, migration routes, and wintering sites of the bar-headed geese from Kyrgyzstan, which represent the westernmost geographical population of the species. As part of a conservation project, which also included reinforcement of the wild population by the release of handreared juveniles, in late summer of 1998, five bar-headed geese, three wild adults and two hand-reared goslings, were fitted with sun-powered satellite transmitters in order to track their movements from Lake Son Kul and Lake Chatyr Kul in Kyrgyzstan. The five individuals contributed very unevenly to the more than 5,000 signals in total that were received from the French ARGOS system: one failed after 8 weeks, while another one was tracked for more than 2 years. The four geese contributing to this study followed three completely different migration routes leading to their wintering areas in Pakistan, India and Uzbekistan, while stopover areas were situated in southern Tajikistan and in western Tibet. Both in autumn and spring the adult birds migrated distances of 1,280-1,550 km in two steps, with stopover periods of 32-46 days (autumn) and 16-23 days (spring). Flight speeds of up to 680 km per actual migration day were recorded regularly, even during crossings of very high summits. A hand-reared juvenile flew non-stop for 790 km to southern Uzbekistan and even visited southernmost Turkmenistan, where the species is very rarely seen. The timing of migration varied considerably between individuals but also for the same individual between years. We compare our tracking results with previous findings (field observations, ring recoveries, and satellite tracking results) and discuss them with respect to migration over high-mountain habitats and a general migration strategy of the species.

Introduction

The bar-headed goose Anser indicus is a waterfowl species of high-mountain wetlands, with a fragmented breeding range from Kyrgyzstan in the west to Manchuria in the east and southward to northern India [START_REF] Hoyo | Bird migration across arid and mountainous regions of middle Asia and Kazakhstan[END_REF][START_REF] Miyabayashi | Atlas of key sites for Anatidae in the east Asian flyway. Tokyo, Wetlands International-Japan and Kuala Lumpur, Wetlands International-Asia Pacific Newton I[END_REF][START_REF] Mckinnon | A field guide to the birds of China[END_REF]. [START_REF] Perennou | The Asian waterfowl census 1987-1991: distribution and status of Asian waterfowl[END_REF] estimated an increasing overall population of approximately 50,000 individuals, while [START_REF] Javed | Tracking the spring migration of a bar-headed goose (Anser indicus) across the Himalaya with satellite telemetry[END_REF] argued that positive population trends may rather be due to increasing counting activities. The species had been listed as near threatened by [START_REF] Collar | Birds to watch-the ICBP world checklist of threatened birds[END_REF] but was deleted from later lists of globally threatened and near threatened bird species [START_REF] Collar | Birds to watch-the ICBP world checklist of threatened birds[END_REF]BirdLife International 2000). In Russia, Kyrgyzstan, and Tajikistan the bar-headed goose has already been regarded as severely endangered for a long time [START_REF] Abdusalyamov | The Fauna of the Tajik SSR. I. Birds[END_REF][START_REF] Selesnev | On the bar-headed goose in Tajikistan (in Russian). In: Gladkov NA (ed) Rare, extinct and less investigated birds of the U.S.S.R. Nauka, Ryasan Swan LW[END_REF][START_REF] Borodin | On flights of bar-headed geese to Taimyr peninsula (in Russian)[END_REF][START_REF] Yakovlev | Current population status of the bar-headed Goose (Anser indicus) in Kirgizia and its preservation[END_REF]Danilov-Danilian 2001, http://birds.krasu.ru).

The only two breeding sites currently left in Kyrgyzstan are Lake Son Kul and Lake Chatyr Kul in the Tien Shan mountains at altitudes of 3,016 m and 3,539 m (Kydyraliev 1967, 1973, Fig. 1).

A total of 170-190 breeding pairs in Kyrgyzstan in the early 1950s had declined to approximately 100 individuals in the late 1990s [START_REF] Yakovlev | Current population status of the bar-headed Goose (Anser indicus) in Kirgizia and its preservation[END_REF]. Human persecution and habitat deterioration are assumed to be the most serious threats to the geese during migration and at wintering sites [START_REF] Javed | Tracking the spring migration of a bar-headed goose (Anser indicus) across the Himalaya with satellite telemetry[END_REF][START_REF] Hoyo | Bird migration across arid and mountainous regions of middle Asia and Kazakhstan[END_REF].

Seasonal movements, migration routes, and wintering grounds of most geographical populations of the bar-headed goose have been little investigated [START_REF] Mcclure | Migration and survival of the birds of Asia[END_REF]. With regard to the Kyrgyz population, it was concluded from ring recoveries (Kydyraliev and Abdusalyamov 1979) that the geese spend winters in the Indus river floodplains in Pakistan, while particular migration habits have remained unknown until now.

Consequently, a joint German-Kyrgyz project, ''Research Programme Bar-headed Goose'', was aimed at (1) the investigation of migration habits of the Kyrgyz population in detail and (2) obtaining evidence that goslings that are hand-reared and released to the wild, as part of a special conservation project in Kyrgyzstan [START_REF] Yakovlev | Current population status of the bar-headed Goose (Anser indicus) in Kirgizia and its preservation[END_REF], would be able to join the wild population and act as recruiters.

Satellite telemetry was regarded as an appropriate tool to achieve those aims. This method has already been applied successfully for a large variety of bird species (see [START_REF] Berthold | Eine aktuelle Gesamtu ¨bersicht. Wissenschaftliche Buchgesellschaft[END_REF] for review). Most instructive with respect to our study were results obtained from brent geese Branta bernicla [START_REF] Gudmundsson | Examining the limits of flight and orientation performance: satellite tracking of brent geese migrating across the Greenland ice-cap[END_REF] and lesser whitefronted geese Anser erythropus [START_REF] Lorentsen | Migration of Fennoscandian lesser white-fronted geese Anser erythropus mapped by satellite telemetry[END_REF]. Satellite transmitters were used for the first time on barheaded geese by [START_REF] Javed | Tracking the spring migration of a bar-headed goose (Anser indicus) across the Himalaya with satellite telemetry[END_REF], who investigated seasonal movements of individuals wintering in Keoladeo National Park, Bharatpur, in Rajasthan, India. The study by [START_REF] Javed | Tracking the spring migration of a bar-headed goose (Anser indicus) across the Himalaya with satellite telemetry[END_REF], as well as a further satellite tracking study undertaken in 2005 [U.S. Geological Survey (USGS) 2006], revealed that birds tracked from wintering grounds in India and Nepal were breeders from Tibet. Until now, no-one has ever tracked bar-headed geese from their breeding grounds to investigate migration features, nor has satellite tracking ever been used for the Kyrgyz population, which is the westernmost population of the species [START_REF] Kear | Bird families of the world. Ducks, geese and swans[END_REF].

Here, we present the results of the satellite tracking of three adult and one juvenile bar-headed geese from the Kyrgyz population, with respect to time patterns and geographical course of their migration cycle.

Methods and material

At the end of July 1998, when we visited Lakes Son Kul and Chatyr Kul to conduct fieldwork for this study, all the observed adult bar-headed geese were flightless due to flight-feather moult. Therefore, three adult wild birds could easily be caught by the use of a small motor boat.

Transmitters were attached to these three adult geese and to two hand-reared goslings which had been transported to Lake Son Kul for release there. For dates and sites of capture/release and status of the tracked individuals, see Table 1. Sexes of the adult wild birds were determined by cloacal inspection. We used microwave platform terminal transmitters (PTT-100) powered by solar panels, which were attached to the birds like back-packs and fixed by thin harnesses of plastic material. The harnesses were enclosed in narrow tubes of smooth Teflon tissue to avoid any destruction of feathers or skin. Great care was taken to ensure that any dislocation of the transmitters was excluded and that the threads were placed well below the body feathers so that the normal protection functions of the plumage were not impaired.

The transmitters weighed approximately 35 g, so that the additional load borne by the geese was about 45 g, including the threads. Altogether, this equipment amounted to approximately 2% of the normal adult bird's body weight and maximally 3% for the young birds (our own data, see also [START_REF] Dunning | CRC handbook of avian body masses[END_REF]. The signals sent continously by the transmitters were received via the French ARGOS satellite system. The periods during which the signals indicated that the birds were alive and their transmitters functioning well lasted between 60 days and 810 days (Table 1). Because one juvenile bird did not leave the site where it was released until failure of its transmitter (14549), data for this bird are not presented here.

The transmitters on the four remaining birds provided different numbers of locations per time unit, while the quality of the location data differed widely according to the particular accuracy classes of the ARGOS system. Those accuracy classes define deviations from real geographical positions as follows: class 3 \ 250 m [±1 standard deviation (SD)], class 2 = 250-500 m (±1 SD), class 1 = between 500 and 1,500 m, class 0 [ 1,500 m (no upper estimate), classes A and B = no accuracy estimation, location accuracy has to be judged by user, and Z = invalid location [START_REF] Argos | User Manual[END_REF].

As has been found in other satellite telemetry studies (e.g. Lorentsen et al. 1998, own data), in our study, out of the total of 5,037 locations received from the four transmitters, only 604 (11.96%) were of ARGOS accuracy classes 1, 2 and 3 (10.16%, 1.41%, and 0.39%). Hence, according to the ARGOS user manual [START_REF] Argos | User Manual[END_REF] almost 90% of the locations were of limited value for the purposes of this study. To obtain more precise locations from the available data set, we used a procedure developed by [START_REF] Kaatz | Der Zug des Weißstorchs Ciconia ciconia auf der europa ¨ischen Ostroute u ¨ber den Nahen Osten nach Afrika[END_REF], who analysed 8,979 cases where real geographical positions, i.e. geographical coordinates by a global positioning system (GPS), of transmitter-tagged German white storks Ciconia ciconia were compared with the respective ARGOS coordinates. He found (1) that the 'accuracy of location estimate' classes of ARGOS in many cases did not reflect the true relationships (see also [START_REF] Hays | The implications of location accuracy for the interpretation of satellite-tracking data[END_REF], and (2) that there were significant correlations between the real accuracy of geographical coordinates and certain parameters provided by each transmitter signal, in particular the parameter 'number of successful plausibility checks' (NOPC). These allow calculation of the extent by which the ARGOS coordinates deviate from those of the real position of the transmitter. Estimating the accuracy of the locations in this way allowed many more locations to be included than when based on ARGOS location accuracy classes exclusively. For the analyses, only locations with an upper limit of 50 km deviation from the positions estimated after [START_REF] Kaatz | Der Zug des Weißstorchs Ciconia ciconia auf der europa ¨ischen Ostroute u ¨ber den Nahen Osten nach Afrika[END_REF] were accepted.

Spatial interpretation of the data was performed with ATLAS GIS software (Strategic Mapping Inc., Santa Clara, CA, USA).

Results

Geographical patterns of migration

The locations plotted in Fig. 2 show three geographical patterns that markedly differ from each other with regard to direction of autumnal departure and the geographical situation of stopover and wintering areas. Five different stopover areas (four in autumn and a further one in spring) and five different wintering areas used by these four birds can be distinguished. These areas, denoted in Fig. 2 by Roman numerals I to X, can be roughly described as follows: All three adult birds (14550,14545,14553) showed a two-step pattern during autumn migration. While 14550 and 14545 left their breeding sites in a south to southwesterly direction to reach areas I and II (Fig. 2a,b), 14553 went in all three recorded seasons in a south-easterly direction to stopover area III in western Tibet (Fig. 2d,e). The first migration step of the latter was approximately four times longer than that of the other two adults. In September 1999 14553 performed several rather long premigration flights before finally leaving the breeding site (Fig. 2e). All three adult birds reached their wintering areas by a second migration step, which led 14545 and 14550 from southern Tajikistan (areas I and II) directly southwards to the middle course of the Indus river in central Pakistan (areas VI and VII), and 14553 from Tibet (areas II and IV) in a westerly direction to north-west India and the Lahore area in Pakistan (areas VIII and IX).

No. 14554, the hand-reared first-year bird, left the release site at Lake Son Kul in mid November in a southwesterly direction to reach area X by a non-stop flight within 1 day (Fig. 2c).

During the stopover periods, which lasted between 32 days and 45 days, the birds generally did not stay at certain sites but switched between suitable sites within a circle of up to about 200 km. This was the case with 14553 (adult male breeder from Chatyr Kul) in Tibet (areas III and IV) as well as with both 14545 (unpaired adult from Chatyr Kul) and 14550 (adult male breeder from Son Kul) in southern Tajikistan (areas I and II).

After arrival at wintering grounds the birds changed their sites rather than staying at one particular site for a longer period. Single long-distance round-flights occurred also in winter: On 16 December 1998, 14550 flew from area IV about 300 km west to Islam Barrage (29.50N 72.31E) close to the Indian border and returned on the same day (Fig. 2a). On 3 January 1999 14554 flew about 550 km south west to the valley of the river Murgab near Kalai-Mor (35.40N 62.32E) in Turkmenistan, and, on 22 February, even to the Karakum Canal near Karamet Niyaz (37.37N 64.32E), in Turkmenistan (Fig. 2c).

Migration movements in spring could be completely tracked only for two birds. While the transmitter of 14545 had already failed in the bird's presumed wintering area (VI), signals were received from 14550 until after the bird had crossed the Hindukush and had reached area II (5 April 1999), where they ceased. In spring 14553 again showed a two-step migration pattern like that in autumn (Fig. 2d,e), although the bird used different geographical routes in the two recorded seasons. Whereas, in spring 1999, it followed precisely the autumn route back, in the following year the bird did not cross the Himalayas but headed north to area V, where it rested for 16 days in April and early May (Fig. 2e).

As in autumn 1998, 14554 went back in spring 1999 by a single non-stop flight from southern Uzbekistan (area X)

Timing and duration of migration

Although the dates of departure from the release sites in autumn 1999 differed between individuals by up to 51 days, the arrival dates at the wintering areas cited above differed only by a maximum of 5 days (Table 2). This time, synchrony was achieved by very flexible lengths of stopover periods, ranging from a maximum of 45 days to almost non-stop migration behaviour as shown by 14554 both in autumn and in spring migration. The maximum year-to-year variation of departure date of the same individual (14553) was 16 days (Table 2).

In spring 1999 the three individuals still tracked at this time left their individual wintering areas at almost the same date in early April (1-3 April), although all three had spent the winter in completely different geographical areas (14554 in Uzbekistan, 14550 in central Pakistan, 14553 in north-eastern Pakistan), and therefore had to use completely different migration routes to get back to Kyrgyzstan. Owing to failure of the transmitter of 14550, the arrival dates at the site of release could be recorded only for two birds. While 14554, the hand-reared juvenile bird, had already reached Lake Son Kul on 2 April, 14553 arrived at its previous year's breeding site (Lake Chatyr Kul) exactly 1 month later. This was because 14553 rested in April 1999 for about 3 weeks in western Tibet (area IV, see above) before heading home.

Again, 14553 may serve as an example that also in spring the individual timing of migration can be highly variable. In 2000 the bird left its wintering area almost 3 weeks later than in the year before, and it arrived at Lake Chatyr Kul 1 week later (Table 2).

The highest daily flight distance was recorded for 14554 while it was migrating to its wintering area in Uzbekistan and back by non-stop flights of 790 km/day and 720 km/day, respectively (Fig. 3a). A similar value was reached by 14545 in autumn 1998 (Table 2,Fig 3a). For 14553, daily flight distances between about 400 km and about 700 km were observed in all of the three (partly) recorded migration cycles (Fig. 3b). As already shown in Fig. 2d, e, this bird completed its comparatively long migration steps to and from the stopover within 6-8 days in autumn and 2-7 days in spring (for the actual migration, see Table 2,Fig 3b), of which several days comprised low daily flight activity and (mostly) only one showed very high flight activity, with distances of 600-700 km covered per day. A similar pattern of daily migration activity was shown by all four recorded individuals (Fig. 3a). Owing to this particular behaviour, the average distances moved by the geese per 'real' migration day were much lower (Table 2).

Discussion

Geographical and time patterns of migration

With reference to 38 recoveries of bar-headed geese ringed in Kyrgyzstan and Tajikistan, Kydyraliev and Abdusalyamov (1979) described roughly the same wintering areas as found by this study in the Indus valley in Pakistan (areas VI and VII). They concluded that birds from both countries are generally migrating directly southwards to common wintering grounds at the middle course of the Indus river in Pakistan. [START_REF] Roberts | The birds of Pakistan, volume 1. Regional studies and non-passeriformes[END_REF] mentioned a further Moscowringed bar-headed goose of unknown origin, which was shot in March 1979 at the Kabul river near Nowshera in Pakistan, approximately 100 km west of the Khyber pass, where, in our study, 14550 passed during spring migration. It may be concluded that spring migration normally leads northwards, following the Indus valley until 35°latitude, from where the geese continue to fly directly north as 14550 did in April 1999.

Field observations confirmed our findings on wintering sites of bar-headed geese in Pakistan. Besides the wellknown wintering sites around the Taunsa barrage, where 14545 and 14550 spent the winter of 1998/1999, [START_REF] Roberts | The birds of Pakistan, volume 1. Regional studies and non-passeriformes[END_REF] points out that ''small numbers regularly come down the Chenab river and sometimes stay on the sandbars upstream of Marala barrage…''. In our study 14553 rested in that area, but it had come from Tibet by crossing the Himalayas (Ladakh Range) from east to west.

Another Moscow-ringed breeder from Lake Son Kul was shot near Lahore, Pakistan, which was regarded by Kydyraliev and Abdusalyamov (1979) as a noteworthy exception. In our study, 14553 was recorded in that region during two winters, so that this ring recovery did not merely indicate an exception but possibly reflected a common wintering area of the Kyrgyz population which still existed in the late 1990s. The same authors assumed northern Kashmir (upper Indus valley and river Gilgit) to be meaningful for Kyrgyz bar-headed geese during spring migration. In our study 14553 rested in this area in spring 2000.

Still unclear remain the origins of 17,000 bar-headed geese wintering in India [START_REF] Ali | Handbook of the birds of India and Pakistan[END_REF][START_REF] Miyabayashi | Atlas of key sites for Anatidae in the east Asian flyway. Tokyo, Wetlands International-Japan and Kuala Lumpur, Wetlands International-Asia Pacific Newton I[END_REF]. Of 26 individuals ringed in winter near Bharatpur, none was reported back [START_REF] Mcclure | Migration and survival of the birds of Asia[END_REF]. The two satellite-tracking studies performed until 2006 revealed that, amongst the winterers in Keoladeo National Park, Bharatpur, are breeders from south-western Tibet [START_REF] Javed | Tracking the spring migration of a bar-headed goose (Anser indicus) across the Himalaya with satellite telemetry[END_REF]USGS 2006). According to our results, the easternmost wintering site of bar-headed geese from Kyrgyzstan was in north-western India, approximately 75 km north of Amritsar (31.35N 74.56E), where 14553 spent two winters. In the latter province a winter population of 1,200 bar-headed geese was counted in 1996, while the species seems to be only a rare visitor in the Indian Punjab (Lopez and Mundkur 1997, The Asian waterfowl census 1994[START_REF] Zhang | The crystal structure of a high oxygen affinity species of haemoglobin (barheaded goose haemoglobin in the oxy form)[END_REF] Results of the coordinated waterbird census and an overview of the status of wetlands in Asia. Kuala Lumpur. Wetlands International, unpublished report). According to our study, these winter populations may be composed at least in part of migrants from Kyrgyzstan.

Winter site fidelity, which is common, e.g., with bean geese in Central Europe [START_REF] Rutschke | Physical conditioning: effect on the myoglobin concentration in skeletal and cardiac muscle of barheaded geese[END_REF], was found for 14553 in this study over 2 years. Further research is needed to confirm this as a general phenomenon also in bar-headed geese.

For the first time, it could be shown that a hand-reared juvenile bar-headed goose (14554) had returned to the site of release after a complete migration cycle. Normally, the future prospects of juveniles, particularly in smaller goose species, strongly depend on extended parental care [START_REF] Prevett | Family and other social groups in snow geese[END_REF] for snow geese Anser caerulescens) or at least on persistent association with family groups [START_REF] Black | Agonistic behaviour in barnacle goose flocks: assessment, investment and reproductive success[END_REF] for barnacle geese Branta leucopsis). Obviously, 14554 could achieve association within a period of almost 4 months between release at Lake Son Kul and leaving from there for autumn migration. Remarkably, 14554 was the only goose in our study that headed in a south-westerly direction to Uzbekistan. As early as April 1973 an adult bar-headed goose was reported from there, and had been ringed in Tajikistan (Kydyraliev and Abdusalyamov 1979). However, as 14554 left Lake Son Kul 6 weeks later than the local breeder 14550, who flew in a southerly direction to Tajikistan, it is likely that 14554 was associated with some waterfowl other than bar-headed geese. Sub-adult bar-headed geese have repeatedly been reported to join greylag geese Anser anser, greater whitefronted geese Anser albifrons and also ruddy shelduck Tadorna ferruginea (Borshonov 1978;[START_REF] Syroechkovskyi | Red data book of Krasnoyarsk county[END_REF], all species breeding or resting at Lake Son Kul as well.

Since the bar-headed goose was not included in the check-list of birds of Uzbekistan and Turkmenistan until 2000 (E. Kreuzberg-Muchina, pers. comm. 2001), it seems unlikely that the recorded migration and wintering behaviour of 14554 was shared by any other bar-headed geese.

Departure directions in autumn migration as well as stopover and wintering areas differ considerably within the Kyrgyz population. This means that endogenous preferences with respect to certain geographical migration patterns in the species hardly exist. The three different geographical patterns found in this study may therefore be interpreted as individually or group-confounded traditions. However, as demonstrated by 14553, the route actually taken by an individual bird may vary from year to year within a broad frame of the preferred geographical migration pattern. Furthermore, the duration of resting periods within the two-step migration pattern (in spring and autumn) of adults varied from year to year by up to 2 weeks. It is noteworthy that the arrival dates at both wintering and breeding grounds varied much less in all recorded birds. It may be concluded that the choice of alternative migration routes, as well as the duration of resting periods, depends on external factors, presumably the currently prevailing weather conditions in the species' high-mountain habitats. For barnacle geese, it has recently been shown that the migratory behaviour of individuals even from the same breeding colony may vary greatly and can be adjusted individually from year to year [START_REF] Eichhorn | Skipping the Baltic: the emergence of a dichotomy of alternative spring migration strategies in Russian barnacle geese[END_REF]).

General migration strategy

As bar-headed geese have been reported to cross the Himalayas at elevations well above 8,000 m (Swan 1970), the species serves as a famous example for the ability of birds to overcome high barriers (e.g. Dolnik 1990; [START_REF] Elphick | Collins atlas of bird migration[END_REF]. It has been shown that bar-headed geese are able to do so due to a higher myoglobin concentration at the beginning of migration than that of other goose species [START_REF] Braunitzer | Molekulare Aspekte der Ho ¨henatmung von Vo ¨geln. Ha ¨moglobine der Streifengans (Anser indicus), der Andengans (Chloephaga melanoptera) und des Sperbergeiers (Gyps rueppellii)[END_REF][START_REF] Rutschke | Physical conditioning: effect on the myoglobin concentration in skeletal and cardiac muscle of barheaded geese[END_REF]. Additionally, they have slightly different haemoglobin structures, which increase the blood's binding affinity for oxygen [START_REF] Zhang | The crystal structure of a high oxygen affinity species of haemoglobin (barheaded goose haemoglobin in the oxy form)[END_REF]; for review see [START_REF] Butler | Flight[END_REF]. Newton (2008) states, with respect to the bar-headed goose, that ''such birds can achieve feats of high-altitude performance that are shown by very few, if any other animals, and can do so without any time to acclimatise''.

The bar-headed geese tracked in this study actually crossed mountain areas regardless of their height and did not avoid even very high summits. 14554 had to overcome the Tien Shan mountains and the Alai range, with summits up to 7,000 m above sea level. 14545 and 14550 crossed the eastern Hindukush range close to Tirich Mir mountain (36.18N 71.55E, 7,750 m above sea level). 14553 rested several times in the westernmost Xinjiang province of China, close to Mt. Godwin Austen (K2) (35.53N 76.32E) at elevations of 5,000 m above sea level. In mid-November, 1998, this bird flew in a 500 km non-stop flight over the Ladakh Range (Himalayas), reaching elevations of 6,000 m, and, in spring 2000, 14553 flew from Skardu, Northern Kashmir (35.18N 75.44E) in a 650 km non-stop flight over the Karakorum mountains, close to the Nanga Parbat mountain (8,126 m above sea level), back to its breeding site at Lake Chatyr Kul.

The environmental conditions under which such movements are undertaken are only predictable in a broad long-term frame, but, in the short-term view, they may get worse in an unforeseeable and dramatic manner. This requires a general migration strategy that allows the birds to escape to more suitable places by very fast flights necessarily leading over a large scale of altitudes in very short times. Barheaded geese have been experimentally proven even to withstand simulated high-altitude conditions of slightly over 12 km [START_REF] Black | Oxygen transport during progressive hypoxia in high-altitude and sea-level waterfowl[END_REF], which, by far, exceeds the tolerance limits of most mammals and many other bird species.

Nevertheless, at lower elevations, topographic features such as the Indus river valley in Pakistan and in northern Kashmir obviously played an important role as geographical guidelines. In contrast to other species of geese of the northern hemisphere, e.g. the barnacle goose [START_REF] Black | Agonistic behaviour in barnacle goose flocks: assessment, investment and reproductive success[END_REF], bar-headed goose migration seems not to be funnelled to small and distinct wintering areas, which was assumed by Kydyraliev and Abdusalyamov (1979) and also by [START_REF] Knystaustas | Birds of Russia. Harper Collins, London Kydyraliev AK (1967) Bar-headed geese (Anser indicus) in the Tian-Shan mountains (in Russian)[END_REF]. Instead, it shows the character of a wide-front migration, as suggested by [START_REF] Javed | Tracking the spring migration of a bar-headed goose (Anser indicus) across the Himalaya with satellite telemetry[END_REF], leading to wintering grounds within a broad belt from central Pakistan to western Myanmar (e.g. [START_REF] Kear | Bird families of the world. Ducks, geese and swans[END_REF]). These findings have been confirmed by the results published by USGS (2006), in which the breeding sites of birds wintering in Nepal were clearly separated from those wintering in the more westerly Bharatpur area of India. In recent times this distribution has increasingly been linked with agricultural crops available for winter feeding (e.g. [START_REF] Choudhury | The bar-headed goose in north-eastern India and Bhutan[END_REF]).

Kyrgyz bar-headed geese rest during migration at sites where conspecifics regularly are resident (e.g. [START_REF] Ming | Breeding ecology of bar-headed goose in Tianshan, Xinjiang[END_REF][START_REF] Lu | Distribution of bar-headed goose in China[END_REF][START_REF] Bishop | Status and distribution of bar-headed geese Anser indicus wintering in south-central Tibet[END_REF]). Hence, they perform a classical leap-frog migration, in which migrating populations of a species overfly resident ones. A pattern like this has been regarded as a special case of differential migration on species' level (Newton 2008).

The series of pre-migration flights of 14553 in September 1999, when the bird flew at least 2,000 km forth and back along its last year's migration route with apparently no gain in terms of true migration, remains unexplained. Similarly, the very fast round-trip flights in winter to sites at remarkable distances from the otherwise small wintering area of 14550 and 14554, can hardly be explained by 'Winterflucht' due to sudden harsher winter conditions, as the birds returned after only 2 days. The occurrence of such undirected long-distance flights documented in this study might explain why bar-headed geese have repeatedly been observed very far away from the traditional migration routes of the species, e.g. in the Bolzshezemelskaya tundra and at the Kanin Peninsula in northwest Russia (e.g. [START_REF] Mineev | Bar-headed geese Eulabeia indica in the Bolshesemelskaya tundra (in Russian)[END_REF], at the Taimyr peninsula in northern Siberia (Borshonov 1978), and in the Baikal region [START_REF] Melnikov | Rare geese species in the Baikal area: distribution and occurrence (in Russian)[END_REF]. Diese satellitentelemetrischen Befunde werden zusammen mit den wenigen vorliegenden Ringfunden sowie mit Feldbeobachtungen hinsichtlich einer generellen Zugstrategie als Anpassung an den Hochgebirgslebensraum der Art diskutiert.

Zusammenfassung

Fig. 1

 1 Fig.1Map of Central Asia, with main high mountain ranges and sites where transmitters were fitted to bar-headed geese. A Lake Son Kul, B Lake Chatyr Kul

(

  I) Lakes and rivers of the southern Pamir range, e.g. Yashil Kul (37.45N 73.00E), Alichur river, southern Tajikistan (II) Rivers Oksu and Pyandsh (37.20N 74.25E) in 'Little Pamir' range, Tajikistan-Afghanistan-China border zone (III) Lake Mindam Tso near Munak Thok (33.09N 81.21E), western Tibet (IV) River valleys between Rutog and Rabang (33.03N 80.29E), western Tibet (V) Indus river valley in the Zaskar mountains., Lake Tsokr and Lake Chumo (33.20N 78.00E), northwestern India

Fig. 2

 2 Fig. 2 Geographical patterns of movements of the four individual bar-headed geese as revealed by satellite tracking. Roman numerals denote different stopover and wintering areas; for details see text

Fig. 3 a

 3 Fig. 3 a Daily flight activity patterns of the four individual barheaded geese from Kyrgyzstan between July 1998 and October 1999; dots below the X-axis indicate the bird present at site of release; the arrow denotes the date of the last signal included in this analysis. b Daily flight activity patterns of 14553, breeding male from Chatyr Kul, between July 1998 and September 2000; dots below the X-axis indicate the bird present at site of release; the arrow denotes the date of the last signal included in this analysis

  Saisonale Raum-Zeit-Muster von vier kirgisischen Streifenga ¨nsen Anser indicus anhand satellitentelemetrischer Daten Der Brutbestand der Streifengans Anser indicus in Kirgistan hat im Laufe der letzten Jahrzehnte dramatisch abgenommen. Da in den Brutgebieten keine erkennbaren Vera ¨nderungen eintraten, werden massive Bejagung abseits der Brutgebiete sowie Verschlechterungen der o ¨kologischen Bedingungen in den Rast-und U ¨berwinterungsgebieten als Hauptfaktoren fu ¨r den Bestandsru ¨ckgang angenommen. Allerdings sind die Zugwege wie auch die Rast-und U ¨berwinterungsgebiete speziell der kirgisischen Population, welche die westlichste innerhalb des geschlossenen Verbreitungsgebietes der Art darstellt, nur sehr bruchstu ¨ckhaft bekannt. Um den diesbezu ¨glichen Wissensstand zu verbessern, wurden im Spa ¨tsommer 1998 im Rahmen eines staatlichen kirgisischen Artenschutzprojektes, das auch Bestandsstu ¨tzungen durch Auswilderung handaufgezogener Go ¨ssel einschließt, drei adulte Wildvo ¨gel und zwei handaufgezogene subadulte Vo ¨gel, mit Satellitensendern versehen. Die besenderten Streifenga ¨nse wurden an den Hochgebirgsseen Son Kul und Chatyr Kul (Tienschan) in die Freiheit entlassen. Von diesen Vo ¨geln konnten u ¨ber das franzo ¨sische ARGOS-System insgesamt mehr als 5.000 Lokalisationen empfangen werden. Die Einzeltiere trugen jedoch in sehr unterschiedlichem Maße zur Datengewinnung bei. Ein Sender fiel bereits nach acht Wochen noch am Freilassungsort aus, ein anderer arbeitete dagegen u ¨ber einen Zeitraum von mehr als zwei Jahren sehr zuverla ¨ssig. Anhand der auf diese Weise gewonnenen satellitentelemetrischen Daten werden in dieser Arbeit die Zugwege und die U ¨berwinterungsra ¨ume sowie die zeitlichen Zugmuster von vier kirgisischen Streifenga ¨nsen im Detail beschrieben. Die vier Streifenga ¨nse, drei adulte Wildvo ¨gel und ein handaufgezogener Jungvogel, nutzten u ¨berraschenderweise sehr unterschiedliche Zugwege und U ¨berwinterungsgebiete. Zwei adulte Vo ¨gel u ¨berwinterten im su ¨dlichen Pakistan, der dritte im indisch-pakistanischen Grenzgebiet nahe Lahore, wa ¨hrend sich der Jungvogel westwa ¨rts wandte und in Usbekistan u ¨berwinterte, wo die Art bisher noch nie nachgewiesen wurde. Im Gegensatz zu den Adulten, die la ¨ngere Rastzeiten im su ¨dlichen Tadshikistan bzw. in Tibet einlegten, erreichte der Jungvogel sein U ¨berwinterungsgebiet in einem Non-Stop-Flug u ¨ber 790 km innerhalb eines Tages. Die adulten Vo ¨gel bewa ¨ltigten ihre etwa 1.280 bis 1.550 km langen Zugwege sowohl im Fru ¨hjahr als auch im Herbst stets in zwei Etappen mit Zwischenstopps von 32-46 Tagen beim Wegzug und 16-23 Tagen beim Heimzug. An Zugtagen entwickelten die Vo ¨gel regelma ¨ßig Fluggeschwindigkeiten von etwa 700 km/Tag, wobei selbst ho ¨chste Regionen von Himalaja, Hindukusch, Tienschan und Karakorum u ¨berflogen wurden. Die zeitlichen Zugabla ¨ufe differierten recht stark zwischen den einzelnen Individuen und beim selben Individuum auch zwischen verschiedenen Jahren.

Table 1

 1 Individual status and periods of data collection for bar-headed geese tracked by satellite transmitters from Lake Son Kul and Lake Chatyr Kul in Kyrgyzstan (M male)

	Individual	Sex	Age/status	Date and site of release	Date of last signal	No. of days with signals
					included in this study	included in this study
	14545	M	Wild adult, no pair-bond	30.07.1998, Chatyr Kul	05.12.1998	135
	14549	?	Hand-reared fledgling	25.07.1998, Son Kul	24.09.1998	None
	14550	M	Wild adult breeder, mated, five goslings	25.07.1998, Son Kul	05.04.1999	252
	14553	M	Wild adult breeder, mated, six goslings	30.07.1998, Chatyr Kul	18.09.2000	810
	14554	?	Hand-reared fledgling	25.07.1998, Son Kul	05.09.1999	411

Table 2

 2 Timing and duration of movement and stopover periods during seasonal migration, and migration distances covered by four bar-headed geese as revealed by satellite tracking

	Individual Autumn						Spring
		Departure	Arrival at	Duration	Distance	Stopover	Speed (km/	Departure	Arrival at	Duration	Distance	Stopover	Speed (km/
		from	wintering	(days)	(km)	days (n)	migration	from wintering	release	(days)	(km)	days (n)	migration
		release site	area				day)	area	site				day)
	14545	01.10.1998 15.11	45	1,360	43	680
	14550	05.10.1998 13.11	38	1,550	32	258	03.04	05.04 b	2	1,020 a	0	510
	14554	15.11.1998 16.11	1	790	0	790	01.04	02.04	1	720	0	720
	14553	25.09.1998 18.11	53	1,280	45	160	02.04	02.05	30	1,280	23	183
		01.10.1999 13.11	43	1,280	37	213	21.04	09.05	18	1,315	16	657
		15.09.2000 18.09 a	3	920	0	307

a Date of final signal from 14553 when the bird had reached area IV in Tibet b Date of final signal from 14550 when the bird had reached north-eastern Afghanistan, near area I to the Cholpon river valley (42.13N 75.19E), which is 50 km north of the release site at Lake Son Kul in Kyrgyzstan.

J Ornithol (2010) 151:703-712

Acknowledgments Thanks to the kind support of the German Max-Planck-Society, Vogelwarte Radolfzell, satellite transmitters became available to conduct this study. We are particularly thankful to Dr. E. Kreuzberg-Muchina, Dushanbe, Dr. V. Toropova, Bishkek, and Prof. A. Kovshar, Almaty, who provided unpublished information on current phenomena in Central Asian waterfowl and submitted additional data on earlier ring recoveries. U. Querner, Radolfzell, continuously assisted the data transfer and commented on the findings with his great experience. Owing to the kind assistance of Dr. J. Shergalin, Tallinn, Russian publications became accessible. A. Bra ¨unlich, Berlin, and A. Globig, Tremt, provided valuable comments on earlier versions of this paper. For catching and marking wild bar-headed geese, as well as for any activities inside areas protected by nature conservation law, official permissions were granted by the state nature conservation authorities of Kyrgyzstan. Many thanks as well to the two anonymous reviewers, who provided invaluable advice for improving earlier versions of the manuscript.


Keywords:

eligible for inclusion. There were 14 males and 27 females. The mean age was 63.76 years (range 24 to 87 years). Minimum follow up was 3 months (12 weeks-43 weeks). Of the 71 procedures, 42 were combined with a blepharoplasty. 62 achieved their desired lid height, contour and symmetry (87.3% success rate). No patients were overcorrected.

Success rate in the phenylephrine positive group (42/71) was 88.1% compared to 100 % in phenylephrine negative group (4/71).

Conclusions

We present a modified approach to ptosis correction via a posterior approach. It has a high success rate and good cosmetic outcome. It is technically straightforward and easy to learn.

Introduction

We describe a surgical technique for ptosis correction in moderate to good levator function involving advancement of the levator aponeurosis via a transconjunctival posterior approach without resection of Muller's muscle. We present our experience of and the results from this method and review the evolution of posterior approach ptosis surgery.

Purpose

To assess the efficacy and predictability of posterior approach white line advancement ptosis repair.

Methods

Retrospective analysis of all patients with primary aponeurotic ptosis undergoing posterior-approach repair using white-line advancement between December 2007 to June 2008. We describe a technique whereby after dissection of the Muller's-conjunctiva composite flap, the levator aponeurosis is advanced with double armed sutures through the white line, then through tarsus and out through skin.

Results

Introduction

Surgery for aponeurotic ptosis has conventionally been directed upon the levator aponeurosis complex, in the majority of cases, through an anterior approach [START_REF] Jones | The cure of ptosis by aponeurotic repair[END_REF] . The transconjunctival route was probably the first method of surgery employed to shorten the levator muscle. [START_REF] Blaskovicz | A new operation for ptosis with shortening of the levator and tarsus[END_REF][START_REF] Blaskovicz | Treatment of ptosis. The formation of a fold in the eyelid and resection of the levator and tarsus[END_REF][START_REF] Agatston | Resection of Levator Palpebrae muscle by the conjunctival route for ptosis[END_REF][START_REF] Berke | A simplified Blaskovics operation for blepharoptosis; results in ninety-one operations[END_REF][START_REF] Iliff | A simplified ptosis operation[END_REF] Posterior approach surgery was further popularised utilising Muller's muscle and tarsoconjunctival resection [START_REF] Fasanella | Levator resection for minimal ptosis: another simplified operation[END_REF][START_REF] Putterman | Müller muscle-conjunctiva resection[END_REF] . Traditionally this posterior approach is reserved for those with minimal ptosis and a positive response to phenylephrine test [START_REF] Fasanella | Levator resection for minimal ptosis: another simplified operation[END_REF][START_REF] Putterman | Müller muscle-conjunctiva resection[END_REF][START_REF] Putterman | Müller's muscle in the treatment of upper eyelid ptosis: a tenyear study[END_REF] .

In 1961 Fasanella and Servat 7 first described their technique initially describing a " resection of Muller's muscle, levator, tarsus and conjunctiva." Histological studies have since shown that the tissue resected consisted of a predominately tarsoconjunctival layer [START_REF] Fox | A modified Fasanella-Servat procedure for ptosis[END_REF][START_REF] Beard | Blepharoptosis repair by modified fasanella servat operation[END_REF] and we now regard the Fasanella servat technique as a tarsoconjunctival resection. Muller's muscle conjunctival resection (MMCR) was first reported by Putterman in 1975 [START_REF] Putterman | Müller muscle-conjunctiva resection[END_REF] , where he dissected Muller's blindly off its bed on the aponeurosis with the aid of a special clamp. This was found to be effective in mild phenylephrine positive ptosis cases. Further modifications to this technique include an open sky MMCR and a conjunctival sparing muller resection [START_REF] Lake | Open sky Müller's muscle-conjunctiva resection for ptosis surgery[END_REF][START_REF] Khooshabeh | Isolated Muller's muscle resection for the correction of blepharoptosis[END_REF] . We describe a surgical technique involving advancement of the posterior surface of the levator aponeurosis (the white-line) via a transconjunctival posterior approach without resection of Muller's muscle. Abraham Werb 29 , working in our unit in East Grinstead, in the1970's was the first to highlight the importance of advancing the white-line via a posterior approach. The whiteline is best described as the distal levator aponeurosis viewed from posteriorly. Although similar techniques have been reported [START_REF] Khooshabeh | Isolated Muller's muscle resection for the correction of blepharoptosis[END_REF] , to our knowledge, the outcomes of advancing the posterior surface of the levator aponeurosis (white-line) through a transconjunctival approach, without resection of Muller's muscle has not been reported.

Materials and Methods

This was a retrospective case series, in which the notes of all patients undergoing surgery for primary involutional aponeurotic ptosis between December 2007 and June 2008 at the Queen Victoria Hospital, East Grinstead, UK, were reviewed. Institutional review board approval was obtained for this retrospective study.

Primary involutional aponeurotic ptosis had been diagnosed in patients with ptosis that was constant in all positions of gaze, moderate to good levator function (levator function > 8mm) and a high or absent skin crease. Patients with congenital, myogenic, neurogenic or traumatic aponeurotic ptosis, inadequate data, including post-operative follow up data with photographs less than 3 months, and those who underwent anterior approach or Muller's muscle excision were excluded.

A total of 41 patients undergoing 71 procedures for involutional aponeurotic posterior approach ptosis repair were identified. Data collected included age, gender, type of ptosis (primary or recurrent), details of any previous ptosis surgery, preoperative upper eyelid margin-reflex distance (MRD), levator function and skin crease height. Response to instillation of phenylephrine 2.5% was also documented preoperatively, with an elevation in lid height deemed a positive result.

Patients were examined postoperatively at 2 weeks and 3 months. Post operative measurements at the last follow up visit were used for comparison. Post operative complications and follow up time were documented. Surgical success was defined as a postoperative MRD of ≥2mm and ≤4.5mm, inter-eyelid height asymmetry of ≤1mm, and satisfactory eyelid contour.

Surgical technique:

All cases were performed under local anaesthesia with 1ml subcutaneous infiltration, both along the skin crease and in the mid-pupil pretarsal region and 0.5ml subconjunctival infiltration upon eyelid eversion using 2% lignocaine with 1:80000 adrenaline (Figure 1A). Most patients also received a bolus injection of intravenous sedation immediately prior to local anaesthetic infiltration, but were alert for the remainder of the procedure.

The desired skin crease was marked and a 4-0 silk traction suture placed in the grey line of the upper eyelid, which was then everted over a Desmarres retractor. Gentle diathermy was applied prior to a conjunctival incision made with a no 15 Bard parker blade along but above the superior border of the tarsus (Figure 1C). Muller's muscle and conjunctiva was dissected off as a composite flap until the white-line, which represents the posterior border of the levator aponeurosis, was identified (Figure 1D). A double armed 5-0 vicryl suture was placed centrally through the posterior belly of the white-line (Figure 1E), in a forehand manner and was then passed through the conjunctival surface of the tarsal plate, 1mm below its superior border (Figure 1F), and then through to the skin (Figure 1G). The suture was captured through the skin in the region of the skin crease. The lid height and contour was assessed after tying this first suture in a bow (Figure 1 H,I) and care was taken to ensure there was no slippage of the suture. If the eyelid position was deemed to be satisfactory, the suture was relaxed and a second suture was placed within 2-3mm lateral to the first in the method described above. Both sutures were then tied. If the lid height was too low after the first suture, it was then relaxed and a second suture was passed higher through the white-line and again through the tarsal plate and skin. If the upper eyelid contour appeared peaked after the first suture, then this was relaxed and a second suture was placed more central to the location of the peak. Using this method of altering the position of the second suture enabled minor adjustments to eyelid height and contour without the undue delay of removing the first suture in the majority of cases. In such situations, the first suture was gently tied so as to act a "support" rather than a "cardinal" suture. In the post-operative period the absorbable sutures were not removed and were left to dissolve spontaneously. Both Muller's and conjunctiva were left to heal spontaneously with no excision of these structures.

In those cases with significant dermatochalasis the procedure was combined with a blepharoplasty. This was carried out prior to eyelid eversion and white-line advancement (Figure 1B). The technique involved either a skin only or skin-muscle blepharoplasty.

The orbital septum and orbital fat were not violated and therefore the anterior surface of the aponeurosis was never exposed. The double armed 5-0 vicryl sutures came out within the wound but through the soft tissue overlying the anterior surface of the tarsal plate. By tying the sutures here they remained buried when skin closure was carried out, usually with continuous 6-0 vicryl rapide.

Results

Patients:

112 ptosis procedures in total during this period of which 71 eyelids of 41 patients were eligible for inclusion. There were 14 males and 27 females. The mean age was 63.76 years ± 14.9 (range 24 to 87 years).

All the procedures were performed for primary involutional ptosis. 41 procedures were excluded as they didn't fit the inclusion criteria. The majority of procedures (84.5%) were simulataneous bilateral ptosis repairs.

Additional upper eyelid/brow procedures performed:

Blepharoplasty was simultaneously performed in 59% (42/71) of ptosis procedures.

Additional endoscopic (2/71) and pretricheal (4/71) brow lifts were carried out.

Preoperative findings:

Mean MRD was 1.32 ± 1.23 59% (42/71) of cases were phenylephrine positive, 6% (4/71) were phenylephrine negative and in 35% (25/71) it was not documented.

Outcomes:

All patients had a minumum of 3 months follow up with a median post operative follow up of 17 weeks and the mean follow up was 20.35 weeks ± 9.6 (12-43).

The overall success rate was 87.3% (62/71). The mean MRD improved to 3.50 ± 1.06 All failures (9/71 eyelids of 9 patients) were due to undercorrection, rather than unsatisfactory contour. The mean time to failure was 4 weeks ± 4.07 (1-14 weeks).

The success rate in the phenylephrine positive group was 88.1% versus 100% in the phenylephrine negative group.

Discussion

We describe a surgical technique for involutional ptosis involving advancement of the posterior surface of the levator aponeurosis via a transconjunctival posterior approach without resection of Muller's muscle or conjunctiva. Overall success rate was 87.3% and all patients achieved a satisfactory contour. Of those that needed further surgery (9/71, 12.7%), all were undercorrected with eyelid height asymmetry and no patients were overcorrected. Our technique differs from previous reports of posterior approach ptosis surgery [START_REF] Lake | Open sky Müller's muscle-conjunctiva resection for ptosis surgery[END_REF][START_REF] Baldwin | Open sky Müller muscle-conjunctival resection in phenylephrine test-negative blepharoptosis patients[END_REF][START_REF] Collin | A ptosis repair of aponeurotic defects by the posterior approach[END_REF] as we do not dissect septum to expose the anterior surface of the levator nor is there any MMCR.

Advancement of a dehisced levator aponeurosis, as described in our technique, combines achievement of a predictable post-operative height expected with aponeurosis reattachment and a predictable normal-looking eyelid contour seen with posterior-approach Muller's muscle resection surgery, yet avoids the need to excise any tissue, namely Muller's muscle or conjunctiva.

Although it would have been useful to compare the phenylephrine response positive with the response negative group the numbers were not sufficient to do so. However the principal of our technique is advancement of the posterior border of the aponeurosis rather than MMCR. One would therefore expect it to be effective irrespective of the response to phenylephrine.

Historically posterior approach ptosis surgery was first described by Blaskovic's 2,3 in 1923. He described a technique of extensive dissection of levator from its surrounding structures prior to its resection. Two sets of sutures were used, the first three were through the levator and the cut edge of the conjunctiva and the second three were "foldforming" sutures through the levator only, and then both were passed anteriorly through the tarsus and skin. This was combined with a tarsectomy in all cases. Posterior approach surgery was further addressed by Agatston [START_REF] Agatston | Resection of Levator Palpebrae muscle by the conjunctival route for ptosis[END_REF] who stated in 1942 that this technique was becoming increasingly popular in the United States. In 1953 Berke [START_REF] Berke | A simplified Blaskovics operation for blepharoptosis; results in ninety-one operations[END_REF] , operating mainly on congenital cases, further simplified Blaskovic's technique. His technique still involved extensive dissection of levator from its surrounding structures however only one set of sutures was used and no tarsal excision was carried out. Following the early papers demonstrating the technique of anterior approach ptosis repair, posterior approach surgery was preferred when focussing on the posterior lamellar structures; tarsus, conjunctiva and Muller's muscle. MMCR is traditionally performed in patients who show eyelid elevation following instillation of topical phenylephrine. In 1961 Fasanella and Servat 7 described a procedure for patients with minimal blepharoptosis in which they applied two curved haemostats to the everted superior tarsus, ran a suture above them, and excised the tissues held by them. The Fasanella-Servat procedure is suitable for mild ptosis and involves a blind approach with predominately tarso-conjunctival resection. In 1975 Putterman and Urist 8 described their technique of MMCR. After separating the Muller's and conjunctiva from the surrounding structures a modified clamp was used and the tissue excised. They found that in most patients the upper eyelid was elevated to a normal level following instillation of 10% phenylephrine and an 8 mm MMCR produced a similar result. The authors recommended a 9mm resection if the phenylephrine test demonstrated a lower eyelid reponse, or a 7 mm resection if the response was higher than desired. At the same time, in Europe a technique for Muller's muscle resection was being popularized by Abraham Werb in our unit. This involved an incision at the superior border of the tarsal plate, dissection and excision of a conjunctival-Muller flap and advancement of the white-line to the tarsal plate and through to the skin crease. To our knowledge, Mr Werb 29 was the first to emphasise the importance of advancing the white-line in this technique. His findings paved the way for others who had worked with Mr. Werb, namely Mr. Collin , to develop techniques in posterior approach levator advancement which are described later in the discussion.

Subsequent modifications to the MMCR technique describe a wide variety of algorithms

to determine the appropriate amount of tissue resection to correct a given degree of ptosis [START_REF] Weinstein | Modification of the Müller's muscle-conjunctival resection operation for blepharoptosis[END_REF][START_REF] Dresner | Further modifications of the Müller's muscle-conjunctival resection procedure for blepharoptosis[END_REF] . Weinstein and Berger [START_REF] Weinstein | Modification of the Müller's muscle-conjunctival resection operation for blepharoptosis[END_REF] suggest that a linear relation may exist between the resultant eyelid elevation and quantity of Muller's muscle resection. This technique begins with 8mm of resection to correct 2mm of ptosis, then adds or subtracts 1mm of resection to affect the final eyelid position by 0.25mm.

Dresner [START_REF] Dresner | Further modifications of the Müller's muscle-conjunctival resection procedure for blepharoptosis[END_REF] describes a further modification of the MMCR technique whereby the amount of resection depends on the response to phenylephrine testing. When phenylephrine testing results in at least 2mm of eyelid elevation, Dresner applied the following algorithm: 4mm of resection for 1mm of ptosis, 6mm of resection for 1.5 mm of ptosis, 10mm of resection for 2mm of ptosis, and 11 or 12mm of resection for >3mm of ptosis.

It appears that widely disparate amounts of resection may yield acceptable surgical results for similar degrees of ptosis. Buchman et al [START_REF] Buchman | Success of Fasanella Servat operation independent of Muller's smooth muscle excision[END_REF] analysed histological specimens from 40 Fasanella-Servat cases which revealed 88% of cases had absent to minimal smooth muscle resection. However these patients had equally successful results in comparison to patients with moderate or large amounts of smooth muscle resection. Perry et al [START_REF] Perry | A new algorithm for ptosis repair using conjunctival Müllerectomy with or without tarsectomy[END_REF] concluded that the success of this technique may result from either vertical posterior lamellar shortening, secondary contractile cicatrisation of the wound, or advancement of the Muller's muscle-levator aponeurosis complex on the tarsus. Perry et al [START_REF] Perry | A new algorithm for ptosis repair using conjunctival Müllerectomy with or without tarsectomy[END_REF] devised a new algorithm based on this theory whereby 9mm of Muller's resection should result in the same elevation that is produced by maximal stimulation with phenylephrine. If phenylephrine testing results in an undercorrection then tarsus was added to the resection.

More recently Lake et al [START_REF] Lake | Open sky Müller's muscle-conjunctiva resection for ptosis surgery[END_REF] and Baldwin et al [START_REF] Baldwin | Open sky Müller muscle-conjunctival resection in phenylephrine test-negative blepharoptosis patients[END_REF] reported their series of open sky MMCR in both phenylephrine test positive and negative groups. Their technique allowed direct visualization of Muller's muscle before its subtotal resection. Double-armed sutures were passed through the Muller's muscle stump, and the upper border of the tarsus through to the skin crease. They suggested that the success with the procedure lay, not from the degree of resection of Muller's muscle, but from the consequent advancement of the levator. As the sutures are passed through the stump of Muller's muscle (which originates at the distal edge of the inferior surface of the levator muscle) and then through the tarsus, one is in effect advancing the levator and attaching it to the tarsal plate and skin crease. This theory was further supported by the success of the procedure in phenylephrine negative patients. [START_REF] Baldwin | Open sky Müller muscle-conjunctival resection in phenylephrine test-negative blepharoptosis patients[END_REF] More recently the authors have also reported a conjunctival sparing technique. [START_REF] Khooshabeh | Isolated Muller's muscle resection for the correction of blepharoptosis[END_REF] Our technique is similar to that reported by Khooshabeh [START_REF] Lake | Open sky Müller's muscle-conjunctiva resection for ptosis surgery[END_REF][START_REF] Khooshabeh | Isolated Muller's muscle resection for the correction of blepharoptosis[END_REF][START_REF] Baldwin | Open sky Müller muscle-conjunctival resection in phenylephrine test-negative blepharoptosis patients[END_REF] in so far as it involves initial dissection of the composite flap of Muller's muscle and conjunctiva with direct visualization of the tissues concerned. However once the posterior border of the levator aponeourosis is identified (the "white-line"), double armed sutures are passed through this white-line and then through the superior tarsus and skin. Only the aponeurosis is therefore advanced and neither Muller's muscle or conjunctiva are excised. Although our current technique involves passing the suture through the epithelial surface of the superior border of the tarsal plate, since presenting this technique to colleagues it has been suggested that gentle diathermy of this area and scraping away of the epithelium with a 15-blade may provide better, more reliable long-term adhesion between the whiteline and the tarsus by elimination of intervening epithelium. We have since included this step and have not encountered any adverse events since doing so.

We know from previous reports [START_REF] Kakizaki | The levator aponeurosis consists of two layers that include smooth muscle[END_REF][START_REF]Upper Eyelid Anatomy: An Update[END_REF] that the levator aponeurosis particularly in asian eyelids comprises two layers, of which the anterior layer is thick with less smooth muscle fibres, and reflects superiorly a few millimeters above the tarsus to become contiguous with the orbital septum. The posterior layer is thin with more smooth muscle fibres, and becomes confluent with the lower one third of the tarsal plate and subcutaneous tissue.

Occasionally, after placement of sutures in what appears to be a white line, there can be an undercorrection of ptosis. We have found that this usually arises from erroneous placement of sutures into the orbital septum (anterior layer of levator) that can occasionally appear as a white line. In such cases, the levator aponeurosis is often significantly thin and can be found by further dissection closer to the conjunctiva beyond a thin Muller's muscle. As Muller's muscle disappears, a thin white line can be identified. Following this, further dissection between this white line and the conjunctiva can reveal a more healthy white sheet, that of the posterior surface of the levator aponeurosis. Placement of sutures into this white sheet, that is to say the healthier posterior surface of levator aponeurosis, will achieve the desired correction by a more effective advancement than simply plicating orbital septum to the tarsus.

The anatomical reasons for the success of Muller muscle conjunctival resection has been a matter of debate for some time now. Increasingly, it has been felt that the success of the procedure is due to advancement of the levator muscle itself, along with the aponeurosis.

The mechanism by which Muller's muscle resection alleviates ptosis would therefore be by transmitting the contraction force of the levator muscle directly to the tarsal plate instead of through its aponeurotic attachment.

Levator advancement through a posterior approach has been previously reported. Collin et al [START_REF] Collin | A ptosis repair of aponeurotic defects by the posterior approach[END_REF] reported a technique in 1979 of reinserting the aponeurosis via a posterior approach. However their technique involved exposing the anterior surface of levator by dissection of the orbital septum and retraction of the preaponeurotic fat pad, effectively converting a posterior approach to the familiar anatomical view of an anterior approach ptosis repair. The initial posterior incision was recommended to be made 2 mm below the superior border of the tarsus in order to enter a tissue plane anterior to Muller's muscle.

Consequently, although conjunctiva was largely preserved, the distal 2mm of Muller's muscle and 2mm of superior tarsus were excised. It is noteworthy that any degree tarsal resection will further contribute to eyelid elevation regardless of muscle resection or levator advancement.

Ichinose et al [START_REF] Ichinose | Transconjunctival levator aponeurotic repair without resection of Müller's muscle[END_REF] Their technique also involved dissection of the orbital septum and exposure of the anterior surface of the levator. The aponeurosis was advanced down and sutured to the frontosuperior part of the tarsus. In addition Muller's muscle was reattached to the superior edge of the tarsal plate. Both of these techniques involved dissection of the levator anteriorly from its surrounding structures before its advancement.

Our technique differs in several ways. We do not dissect septum to expose the anterior surface of levator. We simply pass the sutures through the distal posterior border of the aponeurosis. Our initial incision is just above the superior border of the tarsus as opposed to below, avoiding any excision of tarsus. There is no excision of Muller's or conjunctiva and the Muller's-conjunctiva composite flap we create is simply replaced without the need for resuturing.

Although it has been suggested that advancement of the levator muscle may be the reason for the efficacy of the open sky MMCR [START_REF] Baldwin | Open sky Müller muscle-conjunctival resection in phenylephrine test-negative blepharoptosis patients[END_REF] , this is the first reported series whereby the principle goal is to identify and advance the lower border and posterior surface of the aponeurosis without any exposure of the anterior surface of the levator and without excision of any tissue. Conventional ptosis surgery in the presence of a functioning levator muscle is now predominately on the aponeurosis of the levator complex via an anterior approach. There are some concerns regarding unpredictability of lid height but perhaps more so, contour with this procedure. This is particularly so for the medial undercorrection that occurs more commonly in anterior approach ptosis surgery. [START_REF] Shore | Anatomic changes in involutional blepharoptosis[END_REF][START_REF] Kakizaki | Causes of undercorrection of medial palpebral fissures in blepharoptosis surgery[END_REF] Levator aponeurosis surgery was first advocated by Jones, Quickert and Wobig 1 for the treatment of involutional ptosis. They achieved a success rate of 89% in 57 patients in their original paper describing their technique. [START_REF] Jones | The cure of ptosis by aponeurotic repair[END_REF] In 1979 Anderson and Dixon 25 achieved a 83% success rate in acquired ptosis using the criteria of 1mm or less of residual ptosis as a success. In 1983, Older [START_REF] Older | Levator aponeurosis surgery for the correction of acquired ptosis: analysis of 113 procedures[END_REF] reported a series of cases of aponeurotic repair for acquired ptosis and had a 95 % success rate within 1mm of the desired height in 113 patients with a minumum of 6 months follow up. However, Berlin and Vestal 27 reported for aponeurotic ptosis a 74% success rate within 1mm of desired height in 116 patients. A recent large representative study [START_REF] Mcculley | Outcome and influencing factors of external levator palpebrae superioris aponeurosis advancement for blepharoptosis[END_REF] of anterior levator advancement has shown that 77% of eyelids were symmetrical to within 1mm of the fellow eye after one operation, with 8.8% eyelids requiring further surgery to obtain this outcome. Bilateral cases were twice as likely to require reoperation.

Reports of Muller's muscle resection would suggest a higher success rate compared to that for anterior approach levator advancement. Putterman 9 published a study showing 90% of eyelids achieving within 1.5mm symmetry of the fellow eye. However these results were for predominately mild phenylephrine positive ptosis and if applied to our success criteria would result in a success rate of 75%. Dresner 17 reported 85% of eyelids being within 0.5 mm symmetry. Lake et al [START_REF] Lake | Open sky Müller's muscle-conjunctiva resection for ptosis surgery[END_REF] , describing their open sky technique in phenylephrine positive patients, showed 98% were within 1mm symmetry with the fellow eye. In their second study 14 looking at phenylephrine negative patients all were within 0.5mm symmetry of the fellow eye. Our Muller's muscle-conjunctival sparing technique results in a predictable degree of lid height and contour. In our series there was an 87% success rate and all patients had a good contour. Our criteria for success was somewhat more stringent than other reports in the literature where only an improvement in MRD is defined as success. The procedure can also be combined with a blepharoplasty and this was the case in the majority of our patients. We feel that a posterior approach is still justified in these cases because the septum has not been disturbed following a blepharoplasty. We feel it can be used in the majority of ptosis patients with moderate to good levator function. This technique is easy to teach and learn. The anatomy is easily identifiable, there is minimal dissection and unlike the anterior approach there is little chance of losing your 'anatomical bearings'. We have demonstrated here its predictability and excellent success rate in involutional ptoses with moderate to good levator function.

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  more recently used a similar technique to Collin et al without resecting Muller's muscle. The initial incision was 1mm below the superior border of the tarsus.

Figure 1 :

 1 Figure 1 : Surgical Technique

Figure 2 :

 2 Figure 2: