Juan Carlos

Josep Antoni

Alcover J C Rando
email: jcrando@ull.es

J A Alcover

On the extinction of the Dune Shearwater (Puffinus holeae) from the Canary Islands

Keywords:

Insular ecosystems have been highly punished during the last millennia. Huge numbers of local disappearances and full extinctions of insular birds have been recorded elsewhere, and currently a high number of insular birds are categorised as endangered species. In most of these cases, extinction -or endangerment-is in direct relation with the "aboriginal" and/or imperialist waves of human settlement. Insular bird extinction events, during aboriginal times, have been documented at many archipelagos and isolated islands, such as the Hawaiian Islands, New Zealand, the West Indies or the tropical Pacific Islands. However, in a different way, no bird extinctions could be attributed to the first settlers of the Canary Islands until now. The first accelerator mass spectrometer (AMS) 14 C date of collagen from a bone of the Dune Shearwater Puffinus holeae (3395 ± 30 yr BP), an extinct bird from the Canary Islands, indicates a late Holocene extinction event. The obtained date, together with some features of this bird (large body size, breeding areas situated at very accessible places), and the absence of its bones from the entire archaeological record, suggests that the extinction occurred close to the first human settlement at the islands.

Introduction

A high number of local and total extinctions of seabirds have occurred on islands and archipelagos around the world (see Olson and James 1982b;[START_REF] Quammen | The song of the Dodo: Island Biogeography in an Age of Extinctions[END_REF]Worthy and Holdaway 2002;[START_REF] Rando | Protagonistas de una catástrofe silenciosa: los vertebrados extintos de Canarias[END_REF]Steadman 2006). Otherwise, many of the remaining seabirds on these islands are considered to be endangered at the species or population level (IUCN 2007).

In most cases, extinction -or severe decimation-is directly related to the arrival of "aboriginal" and/or imperialist waves of human settlers (Olson and James 1982b;[START_REF] Quammen | The song of the Dodo: Island Biogeography in an Age of Extinctions[END_REF]Worthy and Holdaway 2002;Steadman 2006). On the Canary Islands, both settlement waves have occurred: the "aboriginal" (known as "Guanches"), arrived from northwest Africa some time before 2000 years ago [START_REF] Navarro | Las primeras etapas del programa de excavaciones en las Cuevas de San Juan y su aportación a la diacronía de la Prehistoria de Canarias[END_REF][START_REF] Atoche | Evidencias arqueológicas del mundo romano en Lanzarote (Islas Canarias)[END_REF]; and the Europeans, who began to colonize the islands in the early 14 th century [START_REF] Castellano | Historia de Canarias[END_REF]. Although the occurrence of insular bird extinctions in aboriginal times has been documented elsewhere (e.g., Hawaii, New Zealand and other Pacific Islands; Olson and James 1982b;[START_REF] James | Prehistoric extinctions and ecological changes on oceanic islands[END_REF]Worthy and Holdaway 2002;Steadman 2006) Both extinct species inhabited the Eastern Canary Islands (Lanzarote, Fuerteventura and surrounding islets, Fig. 1;[START_REF] Martín | Puffinus olsoni n. sp.: nova espècie de baldritja recentment extinguida provinent de depòsits espeleològics de Fuerteventura i Lanzarote (Illes Canàries, Atlàntic Oriental)[END_REF]Walker et al. 1990) and had very different breeding behaviour. Bones of the Lava Shearwater are abundant in caves located at lava fields (known locally as "malpaíses") [START_REF] Martín | Puffinus olsoni n. sp.: nova espècie de baldritja recentment extinguida provinent de depòsits espeleològics de Fuerteventura i Lanzarote (Illes Canàries, Atlàntic Oriental)[END_REF], whereas remains of the Dune Shearwater are frequent at several dune fields (known locally as "jables") (Walker et al. 1990;[START_REF] Michaux | New fossil faunas from Fuerteventura, Canary Islands: Evidence for a Pleistocene age of endemic rodents and shrews[END_REF][START_REF] Marco | Nuevo hallazgo de aves marinas del Pleistoceno de Fuerteventura (Islas Canarias)[END_REF]. In the latter case, egg shells and bones of young individuals have been frequently recovered. Material of the Dune Shearwater (13 bones) has been recorded also at the Mousterian levels (levels 2-4) and a not yet dated upper level (level 1, yielding mixed materials belonging to different periods) of Figueira Brava, a cave placed on the west coast of the Iberian Peninsula [START_REF] Mourer-Chauviré | L"Avifaune pléistocène et holocène de Gruta da Figueira Brava (Arrábida, Portugal)[END_REF].

Like their extinct fellows, the two extant species have different breeding areas. The Little Shearwater breeds on islets and inaccessible cliffs, whereas the Manx Shearwater favours remote scarped hills in laurel forest, sometimes placed several kilometers inland and up to 1000 meters above see level [START_REF] Martín | Puffinus olsoni n. sp.: nova espècie de baldritja recentment extinguida provinent de depòsits espeleològics de Fuerteventura i Lanzarote (Illes Canàries, Atlàntic Oriental)[END_REF]. Both extant species are considered as Endangered Species (EN) under the Canarian Islands Laws, and according to IUCN criteria (Martí and Del Moral 2003).

The extinction of the Lava Shearwater took place during the last Millennium. Its bones, showing evidence of human consumption (i.e., cutting and burning marks), are common in archaeological sites of Fuerteventura [START_REF] Rando | Primeros datos de ornitofagia entre los aborígenes de Fuerteventura (Islas Canarias)[END_REF]. The overlap of the temporal interval for the last occurrence of this species (13-15 th centuries) and the first European presence at the archipelago, suggests a relation between both events (Rando and Alcover 2008). Walker et al. (1990) and [START_REF] Mourer-Chauviré | L"Avifaune pléistocène et holocène de Gruta da Figueira Brava (Arrábida, Portugal)[END_REF] already considered that the extinction of the Dune Shearwater could have been human-mediated, but until now no bones of this species were known from the Canarian Holocene, not even in archaeological contexts (see also [START_REF] Marco | Nuevo hallazgo de aves marinas del Pleistoceno de Fuerteventura (Islas Canarias)[END_REF]. The aim of this paper is to explore the chronology and more probable causes of extinction of the Dune Shearwater.

Materials and methods

During a palaeontological prospection of the western Canary Islands in order to investigate vertebrate extinction events, a humerus of Dune Shearwater of very recent aspect was found at the surface of the top of a little hill at the islet of Lobos (north of Fuerteventura Island; Fig. 1 and Fig. 2). The collagen of this bone was directly dated by accelerator mass spectrometer (AMS) 14 C at the Laboratory of the Royal Institute for Cultural Heritage (Brussels, Belgium).

The 14 C age is expressed as 2 intervals (i.e., p = 95.45 %), and its interpretation is based exclusively on the extreme values of this interval (in order to have a p > 95.45 % indicating that the true age of the dated material is more recent than the lower extreme value of the 2 interval and, independently, it is more ancient than the upper extreme value of the 2 interval) (see Tuggle and Spriggs 2000;[START_REF] Alcover | Bases per al coneixement del contacte entre els primers colonitzadors humans i la naturalesa de les Balears[END_REF]Zilhâo 2001;[START_REF] Ramis | The Chronology of the First Settlement of the Balearic Islands[END_REF][START_REF] Bover | Understanding Late Quaternary Extinctions: the case of Myotragus balearicus Bate 1909[END_REF]Rando and Alcover 2008). Dates derived from the calibration of radiometric results will appear as "cal yr BC".

Radiocarbon samples from species that obtain their carbon from a source (or reservoir) different to atmospheric carbon, such as seabirds, will yield radiocarbon dates excessively old and require the application of a correction factor. The average difference between the radiocarbon date of a terrestrial bone and a marine sample is about 400 radiocarbon years (Stuiver and Braziunas 1993).

The ages were calibrated using the program OxCal v4.0 (Bronk Ramsey 2006), a marine 14 C calibration curve, and a marine reservoir effect of 400 years. Additionally, we present a reservoir correction of ΔR = 275 ± 67 derived from the available closest marine samples to the Canary Islands (four samples from Algarve, Portugal, with a mean reservoir value of 630 years). Reservoir corrections for the world oceans can be found at the Marine Reservoir Correction Database (http://calib.qub.ac.uk/marine/).

Results and discussion

The currently available radiocarbon ages directly obtained on Dune Shearwater material are given in Table 1. The date obtained from the bone of Islet of Lobos (3395 ± 30 yr BP) is around ten times younger than the previous age obtained from an egg shell at Península de Jandía (Fig. 1) (32100 ± 1100 yr BP; Walker et al. 1990). The new date clearly points to a late Holocene extinction event, excluding climatic factors as possible causes for this extinction.

The 2 calibration interval of this date, using the marine 04.14C calibration curve, falls inside the first half of the II millennium BC (1409-1230 cal yr BC). The lower value (1409 cal yr BC) is the most recent available evidence for the occurrence of the Dune Shearwater, and it offers a provisional and minimum estimate for the extinction date. To refine this date, we performed a new calibration using a ΔR = 275 ± 67 obtained from the closest available marine samples to the Canary Islands (four samples from Algarve, Portugal, between 566 and 726 years, with a mean reservoir value of 630 years). The new 2 calibration interval (1159-790 cal yr BC) indicates that the 1409 cal yr BC age for the last occurrence should be considered as a minimum estimate, the extinction having occurred very probably later than 1159 cal yr BC (Table 1). This last 2 calibration interval (1159-790 cal yr BC) is very close to the temporal range proposed for the arrival of the House Mouse Mus musculus to the Canary Islands, which was introduced by humans (756 cal yr BC -313 cal yr AD, Alcover et. al. in press), and fits well with the chronology of the first human settlement of the islands (before 2000 years ago, [START_REF] Navarro | Las primeras etapas del programa de excavaciones en las Cuevas de San Juan y su aportación a la diacronía de la Prehistoria de Canarias[END_REF][START_REF] Atoche | Evidencias arqueológicas del mundo romano en Lanzarote (Islas Canarias)[END_REF]. The closeness between these two temporal intervals could indicate a relation between both events: the aboriginal arrival and the Dune Shearwater extinction.

Three features of the Dune Shearwater might lend support to this hypothesis: (1) Its attractive size, between Cory`s and Manx Shearwaters [thus quite bigger than the Lava Shearwater, which was habitually gathered by the aboriginal populations [START_REF] Rando | Primeros datos de ornitofagia entre los aborígenes de Fuerteventura (Islas Canarias)[END_REF]Rando and Alcover 2008)]; (2) Its very accessible breeding areas, at sand dunes, where nowadays remains of this species are locally very abundant, specially at Península de Jandía (Fig. 1) (Walker et al. 1990;Sanchez Marco 2003). ( 3) Puffinus Shearwaters are highly philopatric, and their colonies are at high risk under introduced predators. Consequently, this species should be very vulnerable to aboriginal hunting. Outside of these breeding sandy areas, the occurrence of bones of this species is extremely rare. The location of breeding areas at islets and in inaccessible places -to humans and other alien predators-seems crucial to explain the survival of the Little and Manx Shearwaters at the Canary Islands [START_REF] Martín | Puffinus olsoni n. sp.: nova espècie de baldritja recentment extinguida provinent de depòsits espeleològics de Fuerteventura i Lanzarote (Illes Canàries, Atlàntic Oriental)[END_REF]Martí and Del Moral 2003).

An analysis of the extinction patterns of land and freshwater birds of the Hawaiian

Islands indicates that prehistoric extinctions showed a strong bias toward larger body sizes, ground-nesting and flightless species, nest type being the primary risk factor for extinction. [START_REF] Boyer | Extinction patterns in the avifauna of the Hawaiian islands[END_REF] clearly identifies two characteristics that are associated with increased aboriginal extinction risk: ground nesting and large body size. In spite of the Dune Shearwater being a seabird, these features derive in a high vulnerability to the first human arrival. The combination of all these facts would explain its quick extirpation after human settlement on the islands. In spite of the intensive field work developed in the last decades, no remains of the Dune Shearwater have ever been found at archaeological sites, whereas bones of other extinct species are abundant (i.e. Lava Shearwater, Lava Mouse Malpaisomys insularis, or Giant Rat from Tenerife Canariomys bravoi; Hutterer et al. 1988, Rando and[START_REF] Rando | Primeros datos de ornitofagia entre los aborígenes de Fuerteventura (Islas Canarias)[END_REF][START_REF] Galván | Orígenes de Buenavista del Norte: de los primeros pobladores a los inicios de la colonización europea[END_REF]Rando and Alcover 2008;Rando et al. 2008). The current absence of evidence for the coexistence of the Dune Shearwater and the aboriginal population should not be interpreted as a taphonomic marker of an absence of contact. It is possible that the first human settlement led to an extirpation of the Dune Shearwater so swift as for not to leave any archaeological or palaeontological trace. Data from other archipelagos indicate that aboriginal loss of species could have occurred within a very short time interval (a century or less) (e.g., Steadman et al. 2002;Steadman 2006).

In an opposite way to other extinct endemic vertebrates, like the Lava Shearwater and the Lava Mouse, that survived until the European contact (Rando & Alcover 2008;Rando et al. 2008), the data presented herein indicate that the Dune Shearwater was still present on the Canary Islands shortly before the human arrival. The absence of later evidence promotes this species as a firm candidate to be considered as directly eradicated by the "Guanches" during the early stages of human settlement of the Canary Islands.

Zusammenfassung

  , no bird extinctions have been definitely attributed to the first settlers of the Canary Islands until now. Four species of the genus Puffinus are known as breeders on the Canary Islands: Manx Shearwater P. puffinus, Little Shearwater P. baroli, Lava Shearwater P. olsoni and Dune Shearwater P. holeae. The former two are extant species, while the last two are extinct taxa. The Lava Shearwater was intermediate in size between the Little and the Manx Shearwater (170-225g and 375-459g respectively, Snow and Perrins 1998), whereas the Dune Shearwater was intermediate between Manx and Cory`s Shearwater Calonectris diomedea (800-1100 g, Snow and Perrins 1998).

Figure 1 .

 1 Figure 1. Map of the Canary Islands showing the location of Lobos Islet and Península de

Figure 2 .

 2 Figure 2. Humeri of P. holeae from Isla de Lobos (A) and Istmo de la Pared, Fuerteventura

Acknowledgments

We thank to Área de Patrimonio del Cabildo de Fuerteventura, especially Nacho Hernández; F. García-Talavera, J.F. Navarro, M. Trias and J. Michaux for information and help in the field work; and Mark van Strydonck and Mathieu Boudin for their help with the radiocarbon dating. This work was supported by Spanish DGICYT Research Project CGL2007-62047/BTE (Cronología y causas de las extinciones de vertebrados autóctonos en Canarias y Baleares: un análisis comparativo. II). We are very grateful to Damià Jaume (Palma de Mallorca) and Eamon Manning (Dublin) for the review of the English. Finally, the comments of two anonymous reviewers contributed to the improvement of the paper.

The authors declare that all the works performed during this project comply with current Spanish laws

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Response to Lingham-Soliar: dinosaur protofeathers: pushing back the origin of feathers into the Middle Triassic

Response to Lingham-Soliar: Dinosaur protofeathers: pushing back the origin of feathers into the Middle Triassic?

Gerald Mayr

Forschungsinstitut Senckenberg, Sektion Ornithologie, Senckenberganlage 25, D-60325 Frankfurt/M., Germany; e-mail: Gerald.Mayr@senckenberg.de In a recent article, Lingham-Soliar (in press) comments on the identification of integumental structures in dinosaurs, concluding that "the scientific methodology is questioned by its failure to make phenomena perceivable by objective means, by questionable rationalizations in critical issues, and by lack of consideration of exceptions to the postulated thesis". A major part of his article focuses on the identification of bristle-like integumental structures on the tail of a psittacosaur fossil from the early Cretaceous of China [START_REF] Mayr | Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus[END_REF]. Lingham-Soliar (in press) harshly criticized this latter study as unsound, but since its results were incorrectly reflected and some of the claims are incorrect, I feel urged to respond to his notes.

A theropod ancestry of birds has been proposed long before any integumental structures were known, and is supported by numerous derived skeletal similarities (see, e.g., [START_REF] Prum | Why ornithologists should care about the theropod origin of birds[END_REF] for a review). Interpretation of the filamentous structures in theropod dinosaurs from the early Cretaceous Jehol Formation as feather homologues is certainly more plausible than Lingham-Soliar"s hypothesis that they represent ossified tendons "after decay and displacement", which is at odds with the fact that ossified tendons are usually tightly attached to the vertebral column, which in the skeletons in question is hardly dissociated.

Psittacosaurs are not closely related to birds, and I agree with Lingham-Soliar that an a priori homologization of the bristle-like tail appendages with avian feathers is not warranted.

Contrary to the impression caused by Lingham-Soliar"s (in press) incomplete citation, however, the psittacosaur bristles have not been interpreted as "protofeathers" by [START_REF] Mayr | Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus[END_REF]. The full paragraph, of which Lingham-Soliar cited only the first sentence, reads "[i]f the "bristles" of Psittacosaurus can be shown to be homologous to the filamentous structures of theropod dinosaurs, similar structures may have already been present in a more comprehensive clade including ornithischian dinosaurs. At present, however, this conclusion would be premature and more detailed investigations on the microstructure and biogeochemistry [...] of the integumentary structures of Psittacosaurus and theropod dinosaurs need to be carried out. Without further evidence, there remains the possibility that the "bristles" of Psittacosaurus are greatly modified scales" (Mayr et al. 2002: 364).

Lingham-Soliar (in press) further questioned interpretation of a dark stripe along these bristles a hollow lumen, whose presence would conflict with an interpretation of these structures as collagen fibers or ossified tendons. I will not discuss whether a lumen represents a "nothingness" (Lingham-Soliar in press) or may eventually be filled with organic matter. However, this stripe, which occurs in the center of numerous bristles, certainly is not an artifact of "some form of diagenesis" (see the detailed description in [START_REF] Mayr | Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus[END_REF]. Lingham-Soliar (in press) noted that "what seriously undermines this thesis for a hollow lumen is the presence of a degraded bristle in one area". However, interpretation of the allegedly "degraded" psittacosaur bristle (Lingham-Soliar in press: fig. 3) is simply incorrect.

Examination of the original fossil would have immediately shown that the bristles are not preserved on the same level as the surrounding matrix, but are elevated relative to it (probably because a sand blasting tool was used for preparation, which more easily removed the soft matrix than the harder fossil remains). The shape of the bristles thus does not necessarily reflect their original shape at the time the fossil was embedded in the sediment, and the frayed margins of some bristles are an artifact of the preparation of the fossil that was done before its acquisition by Forschungsinstitut Senckenberg. In the case of the bristle Lingham-Soliar depicted in the out-of-focus inset of his Figure 3, the outer tube is not "degraded", but was destroyed during preparation.

If Lingham-Soliar had examined the actual specimen, he would have also recognized that the allegedly darker color of the two bristles in the upper right corner of his Figure 3 is a photographic artifact of vignetting, i.e., reduced brightness in the image periphery. Certainly we do not "see the "black" color extending the entire diameter of the "bristle"", and in the original image (Mayr et al. 2002: fig. 3c) the narrow black stripe is clearly visible in at least the lower of these bristles.

Lingham-Soliar (in press) left it to the reader, to decide whether the study of [START_REF] Mayr | Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus[END_REF] represents good science. However, interpretation of subtle features from digital images alone hardly is a step in the right direction, and I also question whether the "Adobe Photoshop Eyedropper tool" constitutes the "up-to-date technology" the author asks for the interpretation of fossils.


Heike Ellrich

Volker Salewski
email: salewski@orn.mpg.de

Wolfgang Fiedler

Morphological sexing of passerines: not valid over larger geographical scales

Keywords: passerines, morphology, sex determination, PCR

Sex determination of birds is important for many ecological studies but is often difficult in species with monomorphic plumage. Morphology often provides a possibility for sex determination, but the characters need to be verified. We tested whether five passerine species can be sexed according to standard morphological measurements applying a forward logistic regression with sex determined by molecular analysis as the dependent variable. Furthermore, we tested whether the results can be used on a larger geographic scale by applying morphological sexing methods gained by similar studies from other regions to our data set. Of the five species of this study only garden warblers could not be sexed morphologically. In the robin 87.2% of all individuals were sexed correctly. For reed warblers, willow warblers and reed buntings the respective values were 77.6%, 89.4% and 86.4%. When the logistic regression functions from similar studies on robins and reed buntings in Denmark and Scotland were applied to the birds from south-western Germany they performed less well compared to the original data set of these studies and compared to the logistic regression function of our own study. The same was the case for willow warblers when a wing length criterion used in Great Britain was applied to the birds of our study. These discrepancies may have several explanations: (1) the models are optimised for the data set from which they were extracted, (2) inter ringer variation in measurements, (3) the use of different age cohorts, (4) different morphology due to different habitat availability around the study site, or, most likely, (5) different morphology due to different migratory behaviour. We recommend that morphological sex differentiation methods similar to this study (1) be only used population specific, (2) only with one age cohort and (3) to adjust the extracted equations from time to time.

Sex determination of birds is important in many ecological studies but sex differentiation is difficult in species with monomorphic plumage even when the concerned individuals are captured. During the breeding season most individuals of many species can be sexed either due to the presence of an incubation patch, most pronounced in females, or the cloacal protuberance in males [START_REF] Drost | Geschlechtsbestimmung lebender Vögel nach der Form der Kloakengegend[END_REF][START_REF] Svensson | Identification Guide to European Passerines[END_REF]). However, during the non-breeding season these characters are invalid. Nevertheless, sexing birds in the non-breeding season is desirable especially for migration studies which analyse differential migration phenomena, i.e. sex specific different migration phenologies, migration routes, wintering areas and wintering ecology with respect to differential habitat use or territorial behaviour.

Many passerine species which are monomorphic in plumage characters show a distinct size dimorphism with males usually being larger compared to females. This size dimorphism is often expressed by longer wings, longer tarsi or a higher body mass [START_REF] Svensson | Identification Guide to European Passerines[END_REF]). These measurements are taken routinely at most ringing stations collecting data for bird migration studies on a large scale [START_REF] Bairlein | Manual of Field Methods[END_REF]. However, although there is often a statistically significant difference in the mean values of a morphological character between the sexes there is also an overlap of varying degrees in the measurements which leads to uncertainties when relying on morphological sex determination alone. Furthermore, to establish morphology as a reliable means for sex determination the respective characters have to be verified by testing them on a sufficiently high number of individuals of explicitly known sex.

In former times researchers sometimes sacrificed a number of individuals of the species under study for sex determination [START_REF] Kalchreuter | Alters-und Geschlechtsbestimmung bei der Rabenkrähe (Corvus corone corone)[END_REF]). An ethically sounder, but nevertheless invasive method is laparotomy when the gonads are checked through a cut into the body cavity of the living bird. The cut heals within several days but care has to be taken to avoid injuries of the liver, the kidney or the digestive tract and there is always the risk of infections [START_REF] Berthold | Die Laparotomie bei Vögeln. Ein Hilfsmittel zur Geschlechstbestimmung und zur Beobachtung des Gonadenzyklus[END_REF]. [START_REF] Morgan | A computer method for resolving mixed normal distributions[END_REF] discussed the possibility to analyse multi-modal distributions of morphological characters to differentiate sexes accordingly. This approach was used e.g. by [START_REF] Catry | Differential migration of chiffchaffs Phylloscopus collybita and P. ibericus in Europe and Africa[END_REF] who separated male and female chiffchaff Phylloscopus collybita based on the bi-modal distribution of wing length. However, although this method is based on sound assumptions there is no independent verification that the individuals classified as males or females do in fact belong to the respective sex cohort. Relatively unproblematic and reliable is taking a blood sample for later molecular sex determination in the laboratory [START_REF] Griffiths | A DNA test to sex most birds[END_REF], but this method is not practicable for all mist-netted birds at field stations where sometimes hundreds of birds are captured daily. Furthermore, in many countries blood sampling is more restricted by animal welfare legislation than trapping and ringing of birds.

However, many recent authors who wanted to verify sex determination by morphological measurements used molecular methods for sex determination [START_REF] Griffiths | A DNA test to sex most birds[END_REF]) of a subsample of the species under study [START_REF] Madsen | Sex-determination of Continental European Robins Erithacus r. rubecula[END_REF][START_REF] Hipkiss | Can migrating Tengmalm's Owls Aegolius funereus be reliably sexed in autumn using simple morphometric measurements?[END_REF][START_REF] Ottvall | Morphological and molecular sex identification of Redshanks Tringa totanus[END_REF]. The sex is then used as the dependent variable in either a discriminant analysis or a logistic regression with a number of morphological measurements as explaining variables to identify the variables associated with the sex of the species under study. In recent years these methods have been used to investigate morphological variables which can be used for sex determination in a number of non-passerine and passerine bird species [START_REF] Madsen | Sex-determination of Continental European Robins Erithacus r. rubecula[END_REF][START_REF] Walton | Sexing first year reed buntings Emberiza schoeniclus using biometrics[END_REF][START_REF] Bertellotti | Determining sex of magellanic penguins using molecular procedures and discriminant functions[END_REF][START_REF] Campos | Sex differentiation of Corn Buntings Miliaria calandra wintering in northern Spain[END_REF][START_REF] Hipkiss | Can migrating Tengmalm's Owls Aegolius funereus be reliably sexed in autumn using simple morphometric measurements?[END_REF][START_REF] Ottvall | Morphological and molecular sex identification of Redshanks Tringa totanus[END_REF], Shealer and Cleary 2007). Although these methods can lead to reliable sex determination of the population under study its general application for the species concerned has two caveats: First, the statistical models are selected to fit a particular sample well. It will therefore fit the sample better than the entire population from which it is drawn or a sample from another population. Second, there are intra-specific differences in morphology due to different migration distances or other ecological factors (temperature, habitat) mainly varying across latitudes. In migratory species those populations with a relatively longer migration distance usually have longer wings compared to populations with relatively shorter migration distance as shown e.g. for the willow warbler [START_REF] Lindström | The autumn migration of Willow Warblers Phylloscopus trochilus in Sweden: results from a nation-wide co-operative project[END_REF] or the blackcap Sylvia atricapilla [START_REF] Fiedler | Ecomorphology of the external flight apparatus of blackcaps (Sylvia atricapilla) with different migration behaviour[END_REF].

In the present study we want to test first whether the sex of first year individuals of five common passerine species which are captured regularly at a constant effort mist-netting site in south-western Germany can be sexed reliably with standard morphological measurements.

For this purpose we used a set of morphological variables in connection with molecular sex determination to verify the validity of morphological sexing. Second, we compare our results with similar studies to test whether morphological sexing is reliable with the same criteria over a larger geographical range. We therefore use only standard measurements which are taken within many studies and thus can be compared with the results of other stations.

MATERIAL AND METHODS

Target species and morphological data

Birds were captured as part of a monitoring programme on the peninsula Mettnau (47.729°N, 8.998°E), Lake Constance, near Radolfzell in south-western Germany. Since 1972 between 30 June to 6 November mist-netting follows standardised methods (details see [START_REF] Berthold | Das "Mettnau-Reit-Illmitz-Programm" -ein langfristiges Vogelfangprogramm der Vogelwarte Radolfzell mit vielfältiger Fragestellung[END_REF]. The early start of the autumn netting season enables the capture of local breeding birds as well as migrating individuals of most species during the entire autumn migration period. The proportions of local birds and of birds on passage are, however, unknown. All captured birds are identified to species, ringed and aged following the criteria given in [START_REF] Svensson | Identification Guide to European Passerines[END_REF] and [START_REF] Jenni | Moult and Ageing of European Passerines[END_REF]. Standard measurements taken are:

(1) feather length i.e. the length of the third outermost primary feather (hereafter: feather) following [START_REF] Berthold | Die Federlänge: Ein neues nützliches Flügelmaß[END_REF]; (2) wing length (hereafter: wing) according to method "maximum length" in [START_REF] Svensson | Identification Guide to European Passerines[END_REF]; and (3) tarsus length (hereafter: tarsus) according to the "alternative method" in Svensson (1992, Fig. 18B). Feather and wing length were measured with a precision of 0.5 mm and tarsus with a precision of 0.1 mm. (4) Body mass is taken with an electronic balance with a precision of 0.1 g. For every bird the fat in the furcula pit was scored with on an ordinal nine digit scale from 0 (no fat) to 8 (entire flight muscle covered with fat) according to [START_REF] Kaiser | A new multi-category classification of subcutaneous fat deposits of songbirds[END_REF]. Furthermore, the thickness of the flight muscle was estimated on an ordinal four digit scale according to [START_REF] Bairlein | Manual of Field Methods[END_REF]. As both estimates showed low variation between individuals (fat scored 0, 1 or 2 in about 87%; muscle scored 1 or 2 in about 88% in all birds respectively) we did not consider fat and muscle scores in the further analyses. Therefore, our study remains comparable to other studies which did also not consider fat and muscle scores (e.g. [START_REF] Walton | Sexing first year reed buntings Emberiza schoeniclus using biometrics[END_REF]. All birds in this study were measured between 30 th June and 27 th September 2007 by H. Ellrich and only those individuals for which all four measurements were available were considered.

Target species were selected according to the a priori expectation of sufficient numbers mistnetted for the analyses based on experience from previous years. Furthermore, only those species were selected in which sex of first year birds before juvenile moult cannot be determined in the hand according to plumage characters. The selected species were (sample size in brackets): robin Erithacus rubecula (94), reed warbler Acrocephalus scirpaceus (147), garden warbler Sylvia borin (79), willow warbler Phylloscopus trochilus (47) and reed bunting Emberiza schoeniclus (44). Morphology of the flight apparatus and body mass of passerines varies with age, i.e. first year birds in many species have shorter wings and a lower body mass compared to older birds (e.g. [START_REF] Alatalo | Why do young passerine birds have shorter wings than older birds[END_REF][START_REF] Schmitz | Autumn migration of Reed Buntings Emberiza schoeniclus in Switzerland[END_REF][START_REF] Markovets | Differential sex-and age-related migration of Bluethroats Luscinia svecica at Eilat, Israel[END_REF]. For some passerine species it has been shown that variation in wing length does also occur between different age cohorts after the first primary moult, i.e. wing length increases with age [START_REF] Merom | Age related changes in wing-length and body mass in the reed warbler Acrocephalus scirpaceus and clamorous reed warbler A. stentoreus[END_REF][START_REF] Dale | Age-related changes in morphological characters in the pied flycatcher Ficedula hypoleuca[END_REF]. Furthermore, the consideration of first year birds only excludes birds in primary moult. The consideration of birds being more than one year old could therefore lead to erroneous results. It is in general not possible to age birds of the species considered in this study when they are more than one year of age, i.e. after their first primary moult. Therefore, only fully grown first year birds were included in the analyses. Every bird was considered only once.

Molecular sexing

Molecular sexing followed [START_REF] Griffiths | A DNA test to sex most birds[END_REF] using a modified PCR amplification of the CHD genes. For the amplification two different primer pairs were chosen: the P2 (5"-TCT GCA TCG CTA AAT CCT TT-3") and the P8 (5"-CTC CCA AGG ATG AG (AG) AA (CT) TG-3") primers [START_REF] Griffiths | A DNA test to sex most birds[END_REF] were used for DNA amplification in the garden warbler, reed warbler, willow warbler and reed bunting and the primer pair 3007 (5"-TAC ATA CAG GCT CTA CTC CT-3") and 3112 (5"-CCC CTT CAG GTT TAA AA-3") was used for amplification in the robin [START_REF] Ellegren | Male-driven evolution of DNA sequences in birds[END_REF]. PCR amplification was carried out in a total volume of 10 µl. The PCR was performed in an Applied Biosystems 2720 Thermal cycler (Applied Biosystems). An initial denaturation step at 94°C for 3 min was followed by 35 cycles of 94°C for 30 s, 50°C for 30 s, 72°C for 45 s and 72°C for 90 s for both primer pairs. A final cooling period of 4°C completed the program. PCR products were separated by electrophoresis on 2% agarose gels. The gel was stained with SybrGold gel stain (Molecular Probes).

Statistics

All morphological variables were tested for a normal distribution with a one-sample Kolmogorov-Smirnov-test. Then a forward conditional logistic regression was used to extract morphological variables which could be used to discriminate between the sexes. In forward conditional regressions removal testing is based on the probability of likelihood-ratio statistics and a variable is entered in the regression when p < 0.05 and a variable is removed when p > 0.1. In the logistic regression sex revealed by the molecular analysis was used as the dependent variable and the morphological measurements feather, wing and tarsus as well as body mass were covariates. The sexes were coded "0" for male and "1" for female for the regression. The goodness-of-fit of the logistic regression was tested with a Hosmer-Lemeshov-test [START_REF] Hosmer | Applied Logistic Regression[END_REF].

With the linear logistic function retrieved from the model

d = α + β 1 x 1 + β 2 x 2 + … + β i x i equation 1
where α is a constant and β 1, 2, …, i regression coefficients of the predictor variables x 1, 2, …, i , an individual bird with certain predictor variables will be classified as being male (d < 0) or female (d > 0). The probability that a bird with a given morphology is female can be estimated according to:

p female = e d /1+e d equation 2
where d is the logistic function and e is the base of natural logarithms, approximately 2.718.

The probability of being male is: p male = 1-p female .

Birds migrating through the study area from more northern latitudes may have a different wing morphology compared to resident conspecifics because of adaptation to different migration distance [START_REF] Leisler | Morphological Consequences of Migration in Passerines[END_REF]Winkler 2003, Fiedler 2005). Therefore, we tested whether day of the season is associated with the morphological measurements with a one-way ANOVA with feather, wing, tarsus and body mass as dependent variables and day of the season (30 June = 1, 1 st July = 2 etc) as a factor.

Due to the low sample size for some species we refrained from splitting the sample to do the analyses with one part of the sample and test its accuracy with the second half of the sample.

SPSS 12.0 was used for all statistical analysis. The accepted significance level was p < 0.05.

RESULTS

Molecular sexing

Molecular sexing revealed that the sample included 54 male and 40 female robins, 68/79 reed warblers, 47/33 garden warblers, 24/23 willow warblers and 24/20 reed buntings. In all species the proportion of the two sexes did not differ significantly from an even ratio (binomial test, p > 0.1).

Morphological sexing

All morphological variables were normally distributed for both sexes in all species (Kolmogorov-Smirnoff test, p > 0.05). In all measurements with significant intra-specific inter-sexual differences male birds showed the higher values, i.e. were the larger sex (Table 1).

A two step forward conditional logistic regression with sex as revealed by molecular sexing as the dependent variable and the morphological variables wing, feather and tarsus as well as body mass as covariates included wing length and feather length (Nagelkerke-R²: 0.669, Hosmer-Lemeshow GOF: p = 0.588) for the robin. With these two variables 87.2% (males:

88.9%, females: 85.0%) of the robins could be sexed correctly with the equation d = 120.585 -1.161*wing -0.655*feather (Table 2, Figure 1a). In reed warblers wing and tarsus were included in a two step logistic regression (Nagelkerke-R²: 0.389; Hosmer -Lemeshow GOF: p = 0.145). With the function d = 72.765 -1.367*tarsus -0.638*wing the sex could be predicted correctly for 77.6% (males: 73.5%, females: 81.0%) of all individuals (Table 2, Figure 1b). In willow warblers only wing was included in the analysis (Nagelkerke-R²: 0.803;

Hosmer-Lemeshow GOF: p = 0.416) and for 89.4% (males: 91.7%, females: 87.0%) of the birds the sex could be predicted correctly with the regression equation d = 97.233 -1.486*wing (Table 2, Figure 1c). For the reed bunting 86.4% (males: 87.5%, females:

85.0%) of all individuals could be sexed correctly according to a one step logistic regression (Nagelkerke-R²: 0.721; Hosmer-Lemeshow GOF: p = 0.981) with the function d = 75.423 -0.98*wing (Table 2, Figure 1d). The garden warbler was the only species for which none of the variables met the conditions for the inclusion in a conditional forward regression. Therefore, it is not possible to sex first year garden warblers using the measurements and the statistical methods applied in this study.

None of the selected variables was significantly associated with capture date (ANOVA: p > 0.05; power on the 0.05 significance level for the robin: wing = 0.855, feather = 0.797; reed warbler: wing = 0.507, tarsus = 0.292; willow warbler: wing = 0.804; reed bunting: wing = 0.595). Therefore, the extracted variables could be used throughout the respective trapping periods.

In addition to the mere classification of a bird as being male or female according to the d values calculated from equation 1, the probability that a bird with a certain combination of morphological measurements is female was estimated according to equation 2. In the robin the morphological criteria for a > 95% probability of belonging to the correct sex was found in 11 (27.5%) of females and in 20 (37%) of males. The respective values for a > 90% probability were 16 (40.0%) for females and 29 (53.7%) for males (Table 3a). In the other species (Table 3b-d) the respective values were for reed warblers: > 95% correct probability for eight females (10.1%) and 3 males (4.4%) and for > 90% correct probability for 14 females (17.7%) and eight males (11.8%), for willow warblers: > 95% correct probability for 13 females (56.5%) and 17 males (70.8%) and for > 90% correct probability for 14 females (60.9%) and 17 males (70.8%) and for reed buntings: > 95% correct probability for four females (20%) and 15 males (62.5%) and for > 90% correct probability for eight females (40%) and 17 males (70.8%). All birds with a > 90% probability to be sexed correctly were in fact sexed correctly.

Comparison with studies from other regions

There are two comparable studies (Madsen 1997, Walton and[START_REF] Walton | Sexing first year reed buntings Emberiza schoeniclus using biometrics[END_REF]) about the sexing of some of our target species from other regions. Additionally, there are several other hints about sexing of these species where the exact methods of verification were not always known.

Below we apply the characters mentioned in former studies to verify their validity for the sample considered in this study. [START_REF] Madsen | Sex-determination of Continental European Robins Erithacus r. rubecula[END_REF] studied robins during the breeding season and during autumn migration in Denmark and used molecular sexing of 138 birds of mixed age to verify morphological sex determination. The author was able to sex 80% of all birds correctly according to wing length based on the results of a logistic regression. According to this regression analysis birds with a wing length ≥ 71mm would be male and birds with a wing length of < 71mm female. When this split was applied the data of this study only 60.6 % of all robins were sexed correctly.

Whereas all males were sexed correctly, 37 (92.5%) out of 40 females were classified as males.

J. Pettersson (cited in Svensson 1992) found that about 60% of "non-adult" "Fenno-Scandian migrant" robins could be sexed at Ottenby, Sweden, by using wing length as the sole separation criterion. Birds with a wing length > 74mm would be males and birds with a wing length < 71mm females although details for the verification of this character are not given.

Applying these characters to the robins considered here revealed that only 34 out of 94 birds fell within the criteria of Pettersson (in Svensson 1992) including 31 males which were all sexed correctly and four females of which one was sexed as a male (wing 75 mm). In total only 35% of all robins could be sexed correctly according to the criteria of Pettersson (in Svensson 1992). [START_REF] Norman | Variations in wing-lengths of Willow Warblers in relation to age, sex and season[END_REF] stated to be able to sex 96% of first year willow warblers caught during April to May in northeast England correctly when assuming that males have a wing length of ≥ 65 mm and females a wing length of < 64 mm. These criteria could be applied to 42 out of 47 willow warblers considered in this study of which 90.5% were sexed correctly. All males (24)

were sexed correctly, but four out of 23 females were sexed as males and 5 females could not be sexed. When applied to all birds of the study 80.9% of all birds could be sexed correctly. [START_REF] Walton | Sexing first year reed buntings Emberiza schoeniclus using biometrics[END_REF] studied reed buntings between July and December from south east Scotland using a logistic regression. With the variables wing and mass 95.4% of all first year birds could be sexed correctly according to the equation: d = 132.31 -1.33*wing -1.58*mass If d < 0 the sex was assigned to be male and if d > 0 the sex was assigned to be female.

Applying this equation to the data set of this study 81.8% of all birds were sexed correctly.

This included 70.8% of correctly sexed males and 95.0% of correctly sexed females.

Comparing the results of [START_REF] Walton | Sexing first year reed buntings Emberiza schoeniclus using biometrics[END_REF] with the results of the logistic regression of our analysis revealed that 4.6% more birds could be classified correctly.

DISCUSSION

In this study we investigated whether the sex of five passerine species can be determined with morphological characters. We restricted our analyses to standard measurements because our intention was to apply potential sex-separating characters to the long time series of this study site and possibly to a wide range of ringing stations working with similar standard methodology. The sex of the measured birds was verified with molecular analyses. In the garden warbler sex determination was not possible with the morphological variables applied in this study and the statistical criteria for the inclusion in a logistic regression. In the other four species a varying number of variables showed significant difference between the sexes.

The sex of the latter four species could be determined correctly with a varying proportion from 77.6% for reed warblers to 89.4% for willow warblers with one (willow warbler, reed bunting) or two (robin, reed warbler) morphological measurements. Therefore, between about 11% and 22% of the measured birds could not be sexed correctly. It is difficult to assess whether this is accurate enough to be used in further analyses. This question will depend mainly on the questions addressed and on the species considered.

There are several possibilities which may improve the potential application of studies using an approach like the one presented here. First, a repetition of a similar analysis, but applied to a higher number of birds may improve its accuracy. However, on an inter-specific level a larger number of individuals analysed is not related to a higher proportion of correctly sexed birds. This study included 147 reed warblers but only 47 willow warblers, but the proportions of correctly sexed birds was 77.6% and 89.4% respectively. Second, the proportion of correctly sexed birds can be increased when only a subset of the sample is used according to the most discriminating values of the respective morphological variables. When the probability that a bird with certain character values belongs to a certain sex was >90% all respective individuals were in fact sexed correctly. Therefore, these combinations of character values could be used to select a subsample of individuals for studies that need to analyse sex specific traits. This approach may, however, include the caveat that only individuals at the extreme range of certain variables are considered. For example, body size, amongst other factors, may be an indicator of the condition of an individual. In size dimorphic species with e.g. larger males, an inter sexual overlap of characters may lead to the mere consideration of "large" males and "small" females, i.e. a preference for the individuals with high condition in one sex over those individuals with low condition in the other sex with consequently spurious results.

The study showed that morphological sex determination according to logistic regression functions could only be applied with care between study sites and generally performs with less accuracy between sites. If all birds of the target species were considered then the application of morphological characters from other studies performed less well than testing our own data. This may have been for several not mutually exclusive reasons. First, the statistical analysis itself influences the results. In a logistic regression model variables are selected to fit the particular data set well. The fit will therefore be less good when a different data set, either from the same or a different population, is used. This may be the reason for smaller differences like in the application of the criteria of [START_REF] Walton | Sexing first year reed buntings Emberiza schoeniclus using biometrics[END_REF] to our data set of reed buntings, but we assume that differences in model fit are unlikely to cause the difference in explanatory power of approximately 20% of the models as in the robin. Second, inter-ringer variation in measurements may influence the results. The occurrence of interringer variances in standard measurements has been shown in a number of studies [START_REF] Nisbet | Statistical comparison of wing-lengths measured by four observers[END_REF][START_REF] Berthold | Die Federlänge: Ein neues nützliches Flügelmaß[END_REF][START_REF] Gosler | The field determination of body size and condition in passerines: a report to the British Ringing Committee[END_REF]. Nevertheless, assuming that the ringers in all studies were experienced, the differences between the means of the measurements according to internationally standardized methods should be low [START_REF] Nisbet | Statistical comparison of wing-lengths measured by four observers[END_REF]). Therefore, it seems unlikely that inter-ringer variation in measurements can account for the magnitude of some of the differences of the explanatory power of certain variables between this study and those from other sites (possible exception: see below). Third, the inclusion of different age cohorts in the analyses may lead to biased results. It has been shown

for many passerine species that first year birds have shorter wings than older birds (e.g. [START_REF] Alatalo | Why do young passerine birds have shorter wings than older birds[END_REF][START_REF] Tiainen | Wing shape variation of Finnish and Central European Willow Warblers Phylloscopus trochilus and Chiffchaffs P. collybita[END_REF][START_REF] Norman | Juvenile wing shape, wing moult and weight in the family sylviidae[END_REF]). Additionally, it has been discussed that wing length of passerines increases with age [START_REF] Smith | Song sparrows grow and shrink with age[END_REF][START_REF] Dale | Age-related changes in morphological characters in the pied flycatcher Ficedula hypoleuca[END_REF] and may decrease again in old age [START_REF] Møller | Senescence in a short-lived migratory bird: age-dependent morphology, migration, reproduction and parasitism[END_REF]. The inclusion of different age cohorts as in the study of [START_REF] Madsen | Sex-determination of Continental European Robins Erithacus r. rubecula[END_REF] on robins could be the reason for the strong sex bias when the discriminating criteria were applied to the birds of our study.

However, due to the inclusion of adult robins into the sample the mean wing lengths should be longer compared to a sample where only first year birds are considered. Additionally, the same effect should appear if robins from northern Europe are expected to have longer wings than birds from Central Europe. However, [START_REF] Madsen | Sex-determination of Continental European Robins Erithacus r. rubecula[END_REF] noted a distinctly shorter mean wing lengths for both sexes (males: 72.2 ± 1.32 sd; females: 69.8 ± 1.54 ± sd) than we did in south-western Germany (Table 1). Madsen"s wing length data is also lower than comparable data from Scandinavia [START_REF] Pettersson | Rödhakens Erithacus rubecula höstflyttning vid Ottenby[END_REF]. Due to these differences all males of our study are sexed correctly according the criteria of [START_REF] Madsen | Sex-determination of Continental European Robins Erithacus r. rubecula[END_REF], but the great majority of females are also sexed as males. Although the same measurement technique was used in both studies (method "3" according to Svensson 1992), a ringer bias cannot be excluded. Fourth, birds considered at different sites may differ in their morphology (see e.g. [START_REF] Hanski | Morphological variation in sympatric and allopatric populations of European Willow Warblers and Chiffchaffs[END_REF] for willow warblers). This may either be due to different habitats surrounding the study site as morphology of a number of passerine species has been found to differ between habitats [START_REF] Lundberg | Biometry, habitat distribution and breeding success in the pied flycatcher Ficedula hypoleuca[END_REF][START_REF] Michalak | Inter-habitat morphometric differentiation of male Willow Warblers Phylloscopus trochilus[END_REF][START_REF] Blondel | Coping with habitat heterogeneity: the story of Mediterranean blue tits[END_REF], or to different migratory behaviour.

There are no data on habitat types of the study sites considered, thus this influence cannot be excluded, but [START_REF] Cuadrado | Wing length criteria for sex determination of Robins Erithacus rubecula wintering in southern Spain[END_REF] explained the low probability of sexing wintering robins correctly in Spain with the inclusion of birds with different migratory behaviour into the study. It has been discussed that migration distance is linked with wing length [START_REF] Leisler | Morphological Consequences of Migration in Passerines[END_REF] and [START_REF] Fiedler | Ecomorphology of the external flight apparatus of blackcaps (Sylvia atricapilla) with different migration behaviour[END_REF] showed on an intra-specific level that blackcaps Sylvia atricapilla with increasing migratory distances have longer wings. Therefore, site specific morphology as an adaptation to different migratory behaviour is presumably the main reason why morphological sexing can not be applied over larger geographic scales. Assuming longer wings in populations from more northerly latitudes because of longer migration distances the wing length which splits the sexes should be higher than in more southern populations and consequently more shorter winged females should be sexed correctly and more males incorrectly. However the opposite is the case when applying the criteria of Pettersson (in Svensson 1992) and of [START_REF] Norman | Variations in wing-lengths of Willow Warblers in relation to age, sex and season[END_REF] to the robins and the willow warblers of this study respectively. We have no information how the sex of the robins studied by Pettersson (in Svensson 1992) was verified which makes it difficult to discuss the differences compared to our study. With respect to willow warblers British birds spend the non-breeding season in West Africa [START_REF] Wernham | The migration atlasmovements of the birds of Britain and Ireland. T & AD Poyser[END_REF] and recoveries of willow warblers ringed in southern Germany indicate similar non-breeding areas [START_REF] Zink | Der Zug europäischer Singvögel I[END_REF]. Therefore, the distance of south German willow warblers to their non-breeding areas may be even longer compared to British willow warblers although the latter breed at higher latitudes. Furthermore, the birds of this study could include some migrants from more north-eastern breeding areas with an even longer migration distance.

In conclusion, we showed that in four out of five investigated passerine species the sex could be determined with varying accuracy. For some species the proportion of correctly sexed individuals was low and may not be sufficient to draw sound inferences when applied to further analyses e.g. for the investigation of differential migration patterns. Furthermore, it is

shown that results from one study site will cause spurious results when applied at another study site for a variety of mutually non-exclusive reasons. As morphology of species at a given site may also vary with time (Nowakowski 2000, Salewski et al. in press) we recommend the use of morphological sex differentiation methods similar to this study only on a population specific basis, only with one age cohort, only birds at the same stage of feather wear (feathers in spring may be more worn compared to feathers in autumn or vice versa) and to adjust the extracted equations from time to time.

ZUSAMMENFASSUNG

  Kriterien zur morphologischen Geschlechtsbestimmung beiSingvögeln sind nicht über größere geographische Räume hinweg anwendbar Die korrekte Geschlechtsbestimmung von Singvögeln ist für viele Studien erwünscht, aber bei monomorphen Arten oft schwierig. Die Geschlechtsbestimmung anhand morphologischer Merkmale bildet einen guten Ansatz, muss aber anhand anderer Methoden verifiziert werden. Wir prüften bei fünf Singvogelarten mit Hilfe einer logistischen Regression, in der das durch molekulare Methoden bestimmte Geschlecht der untersuchten Vögel als unabhängige Variable einging, ob eine morphologische Geschlechtsbestimmung anhand von Standardmaßen (Flügel-, Feder-und Tarsuslänge, Gewicht) durchgeführt werden kann. Darüber hinaus untersuchten wir, ob durch ähnliche Untersuchungen ermittelte Geschlechtsbestimmungskriterien auch auf die von uns untersuchten Vögel mit gleicher Genauigkeit angewandt werden können. Von den untersuchten Arten ließen sich nur bei der Gartengrasmücke keine morphologischen Variablen ermitteln, die der Geschlechtsbestimmung dienen können. Beim Rotkehlchen konnte bei 87,2% der Vögel das Geschlecht korrekt bestimmt werden. Für Teichrohrsänger, Fitis und Rohrammer waren die entsprechenden Werte 77,6%, 89,4% und 86,4%. Wenn die Funktion einer logistischen

Figure 1 .

 1 Figure legend:

Table 1 :

 1 Morphological measurements of five passerine species. Shown is the number of birds considered (male/female), mean (significant intersexual differences according to a 2-tailed t-test with equal variances are indicated with * when p<0.05 and *** when p<0.001) and standard deviation (sd) of body mass, tarsus length, wing length and feather length of both sexes (determined by molecular sexing).

	Species		Mass	Tarsus		Wing	Feather
			male	female	male	female	male	female	male	female
	Robin	Mean	16.31	15.98	25.16	25.18	74.68*** 72.24*** 55.69*** 53.48***
	(54/40)	Sd	0.93	0.78	0.68	0.74	1.44	1.12	1.27	1.02
	Reed Warbler	Mean	11.16	11.24	22.63*** 22.02*** 66.58*** 65.27*** 50.48*** 49.40***
	(68/79)	Sd	0.63	0.62	0.65	0.56	1.36	1.32	1.383	1.32
	Garden Warbler	Mean	18.76	18.82	19.78	19.63	78.16	77.76	58.53	58.24
	(47/33/)	Sd	1.67	2.08	0.70	0.70	1.86	2.58	1.50	2.00
	Willow Warbler	Mean	8.88*	8.13*	19.46*** 18.51*** 67.69*** 63.39*** 50.90*** 47.76***
	(24/23)	Sd	0.69	0.85	0.58	0.55	1.55	1.55	1.50	1.48
	Reed Bunting	Mean	18.20*	16.55*	19.62*	18.96*	79.56*** 74.90*** 60.35*** 56.20***
	(24/20)	Sd	1.58	1.29	0.64	0.71	1.97	1.96	1.69	1.94

Table 2 :

 2 Results of a logistic regression with sex as the dependent variable and all morphological characters as covariates. Shown are the B values (± SE) and the results of Wald statistics of the characters and the constant of the model for each species.

	Species		B	SE	Wald	p
		Mass	-0.214	0.355	0.364	0.546
		Tarsus	0.760	0.462	2.709	0.100
	Robin	Wing	-1.376	0.488	7.952	0.005
		Feather	-0.588	0.421	1.951	0.162
		Constant	117.070	26.317	19.789	<0.001
		Mass	0.115	0.334	0.119	0.731
		Tarsus	-1.362	0.345	15.629	<0.001
	Reed
		Wing	-0.513	0.222	5.344	0.021
	Warbler
		Feather	-0.165	0.199	0.685	0.408
		Constant	71.331	13.868	26.457	<0.001
		Mass	0.086	0.138	0.394	0.530
		Tarsus	-0.324	0.376	0.740	0.390
	Garden
		Wing	-0.070	0.207	0.114	0.735
	Warbler
		Feather	0.002	0.261	<0.001	0.995
		Constant	9.775	9.374	1.087	0.297
		Mass	-0.118	0.703	0.028	0.867
		Tarsus	-0.618	1.015	0.371	0.543
	Willow
		Wing	-1.389	0.626	4.917	0.027
	Warbler
		Feather	-0.034	0.542	0.004	0.956
		Constant	105.207	35.368	8.849	0.003
		Mass	0.152	0.406	0.139	0.709
		Tarsus	-0.786	0.831	0.894	0.344
	Reed
		Wing	-0.389	0.460	0.713	0.399
	Bunting
		Feather	-0.776	0.568	1.871	0.171
		Constant	87.646	26.341	11.071	0.001

Table 3 :

 3 Probability of a bird with a certain combination of morphological variables being female. Bold line separates probabilities indicating that a bird is female (p>0.5) or male (p<0.5). Dark grey shading: indicates >95% probability of being female (p>0.95) or male (p<0.05). Light .924 0.898 0.865 0.823 0.772 0.711 0.642 0.566 0.486 0.408 0.333 0.267 0.209 0.161 0.122 22.2 0.927 0.902 0.870 0.830 0.780 0.721 0.652 0.577 0.498 0.419 0.344 0.276 0.217 0.167 0.127 0.096 22.4 0.906 0.875 0.836 0.788 0.730 0.662 0.588 0.509 0.430 0.354 0.285 0.224 0.174 0.133 0.100 0.075

	grey shading: indicates ≥90% probability of being female (p>0.900) or male (p≤0.100).
	a) Robin
									Feather-length [mm]
			51.5	52.0	52.5	53.0	53.5	54.0	54.5	55.0	55.5	56.0	56.5	57.0	57.5	58.0
		70.0	0.996	0.995	0.993	0.990	0.986	0.981	0.974	0.964	0.951	0.933	0.909	0.879	0.839	0.790
		70.5	0.993	0.991	0.987	0.982	0.976	0.967	0.954	0.938	0.915	0.886	0.849	0.802	0.745	0.678
		71.0	0.988	0.984	0.977	0.969	0.957	0.942	0.921	0.894	0.858	0.814	0.759	0.694	0.620	0.541
		71.5	0.979	0.971	0.960	0.946	0.926	0.901	0.867	0.825	0.772	0.710	0.638	0.559	0.478	0.397
		72.0	0.963	0.949	0.931	0.907	0.876	0.835	0.785	0.725	0.655	0.578	0.496	0.415	0.339	0.270
		72.5	0.936	0.913	0.883	0.845	0.797	0.739	0.672	0.596	0.515	0.434	0.355	0.284	0.223	0.171
		73.0	0.891	0.855	0.809	0.753	0.688	0.613	0.534	0.452	0.373	0.300	0.236	0.182	0.138	0.104
	[mm] Wing-length	73.5 74.0 74.5 75.0 75.5	0.820 0.719 0.589 0.445 0.309	0.767 0.648 0.508 0.366 0.244	0.703 0.570 0.426 0.294 0.189	0.631 0.489 0.349 0.231 0.144	0.552 0.408 0.278 0.178 0.108	0.470 0.332 0.218 0.135 0.080	0.390 0.264 0.167 0.101 0.059	0.316 0.205 0.126 0.075 0.043	0.250 0.157 0.094 0.055 0.032	0.193 0.118 0.070 0.040 0.023	0.147 0.088 0.051 0.029 0.017	0.111 0.065 0.038 0.021 0.012	0.082 0.048 0.027 0.015 0.009	0.061 0.035 0.020 0.011 0.006
		76.0	0.200	0.153	0.115	0.086	0.063	0.046	0.034	0.025	0.018	0.013	0.009	0.007	0.005	0.004
		76.5	0.123	0.092	0.068	0.050	0.036	0.027	0.019	0.014	0.010	0.007	0.005	0.004	0.003	0.002
		77.0	0.073	0.054	0.039	0.029	0.021	0.015	0.011	0.008	0.006	0.004	0.003	0.002	0.002	0.001
		77.5	0.042	0.031	0.022	0.016	0.012	0.008	0.006	0.004	0.003	0.002	0.002	0.001	0.001	0.001
		78.0	0.024	0.017	0.013	0.009	0.007	0.005	0.003	0.002	0.002	0.001	0.001	0.001	0.000	0.000
		78.5	0.014	0.010	0.007	0.005	0.004	0.003	0.002	0.001	0.001	0.001	0.001	0.000	0.000	0.000
		79.0	0.008	0.006	0.004	0.003	0.002	0.001	0.001	0.001	0.001	0.000	0.000	0.000	0.000	0.000

ACKNOWLEDGEMENTS

We thank all volunteers at the Mettnau ringing station in 2007 that made this study possible. HE thanks J. Martens, Johannes-Gutenberg-University, Mainz, for his support and H. van Noordwijk for the help in the laboratory. We thank G. Segelbacher, Greg Conway and an unknown referee for discussing our manuscript. R. Holland kindly improved our English. All work related to the handling of birds (current permit No. 56-8853.17/01, Regierungspräsidium Freiburg) and blood sampling (permit O-05/39a, Regierungspräsidium Freiburg) was done in compliance with German law.


Alain Jacot
email: jacot@orn.mpg.de

Mihai Valcu

Bart Kempenaers

Within-season divorce in Blue Tits ()

Keywords: blue tit, Cyanistes caeruleus, intrasexual competition, within-season divorce, rapid mate switching

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Introduction

The mating system of most bird species is characterized by social monogamy [START_REF] Lack | Ecological adaptations for breeding in birds[END_REF] where breeding partners are occasionally changed between breeding attempts. Re-mating with a new partner can be triggered by the death of the former partner (i.e. "widowed re-mating") or by the reduced probability of finding the former partner [START_REF] Cezilly | Re-mating between and within breeding seasons in the greater flamingo Phoenicopterus ruber roseus[END_REF][START_REF] Ens | Mate fidelity and divorce in monogamous birds[END_REF].

However, re-mating can also result from a strategic decision of at least one partner, when both individuals are still alive (i.e. divorce [START_REF] Choudhury | Divorce in birds -a review of the hypotheses[END_REF]). In many bird species, divorce has been discussed as a reproductive strategy whereby individuals can maximize their own fitness by gaining direct or indirect (genetic) fitness benefits from mating with a new partner [START_REF] Ramsay | Divorce and extrapair mating in female black-capped chickadees (Parus atricapillus): separate strategies with a common target[END_REF][START_REF] Green | Mate choice in the brown thornbill (Acanthiza pusilla): are settlement decisions, divorce and extrapair mating complementary strategies?[END_REF][START_REF] Moody | Divorce in common murres (Uria aalge): relationship to parental quality[END_REF], although the behavioural processes that ultimately result in a divorce have rarely been studied. Models predict that individuals should only divorce if the expected future gain in fitness with the new mate will exceed the expected gain with the old mate [START_REF] Mcnamara | Divorce rates in birds: Predictions from an optimization model[END_REF][START_REF] Mcnamara | An ESS model for divorce strategies in birds[END_REF]. The causes of divorce and the fitness consequences of divorcing individuals have been investigated in several bird species [START_REF] Jeschke | Mortality and other determinants of bird divorce rate[END_REF]. For example, great tits Parus major that raised an experimentally reduced brood in the previous year were more likely to mate with a new partner [START_REF] Linden | Divorce in great tits -chance or choice -an experimental approach[END_REF], which indicates that previous breeding performance may influence the propensity to divorce. In oystercatchers, Haematopus ostralegus, divorcing individuals increased their lifetime reproductive success, but the fitness gain depended on the status of their new mating partner [START_REF] Heg | Fitness consequences of divorce in the oystercatcher, Haematopus ostralegus[END_REF]. In blue tits, Cyanistes caeruleus, breeding success of female divorcees decreased in one study [START_REF] Pampus | Pair bond and breeding success in blue tits Parus caeruleus and great tits Parus major[END_REF], whereas two other studies indicated potential fitness benefits of divorce [START_REF] Blondel | High divorce rates in Corsican blue tits: how to choose a better option in a harsh environment[END_REF][START_REF] Valcu | Causes and consequences of breeding dispersal and divorce in a blue tit, Cyanistes caeruleus, population[END_REF]. In a long-term study, [START_REF] Valcu | Causes and consequences of breeding dispersal and divorce in a blue tit, Cyanistes caeruleus, population[END_REF] have shown that once territory quality and breeding dispersal were taken into account, both males and dispersed females increased their breeding success. The same study also suggested that divorce can be independent of mating decisions by the former pair members and may be triggered by female-female competition, whereby a dominant female evicts the former mate.

These studies show that fitness consequences of divorce are often complex and may partly depend on ecological factors that affect the costs of mate searching and mate sampling as well as the probability to establish or find a vacant territory. A crucial, however largely unexplored factor affecting the probability to find an optimal partner and territory could be the time within the breeding cycle when divorce and re-mating happens. Fitness costs may arise whenever there are timeconstraints to find an optimal mate, i.e. divorcing shortly before the onset of the breeding season should incur higher costs compared to divorce and re-mating during the non-breeding season.

Whenever environmental conditions change rapidly, thereby altering breeding conditions and the availability of nesting sites or mating partners shortly before the breeding season, individuals may respond flexibly and adopt reproductive strategies to ensure some breeding success. Such strategies may involve intraspecific brood parasitism [START_REF] Gowaty | Nestbox availability affects extra-pair fertilizations and conspecific nest parasitism in eastern bluebirds, Sialia sialis[END_REF][START_REF] Jacot | Experimental nest site limitation affects reproductive strategies and parental investment in a hole-nesting passerine[END_REF], paternity gain via extra-pair copulations [START_REF] Kempenaers | Extra-pair paternity and the reproductive role of male floaters in the tree swallow (Tachycineta bicolor)[END_REF], or the re-mating with a new partner.

Whenever divorce and re-mating occurs during the breeding season, it is also referred to as rapid mate switching. This type of divorce may be more common than currently reported (e.g., 14% in a study on blackbirds Turdus merula, Wysocki 2004), but its causes and consequences have received relatively little attention. One cause may be predator-induced mortality of one partner, where the other individual has to find a new mate, as suggested in a field study in European starlings Sturnus vulgaris [START_REF] Pinxten | Extra-pair paternity and intraspecific brood parasitism in the european starling, Sturnus vulgaris -evidence from DNA fingerprinting[END_REF]. In contrast, within-season divorce may also be initiated when the old partner is still alive. In a correlative and experimental study, it has been shown that female blackcapped chickadees Parus atricapilla were more likely to divorce their current partner shortly before the breeding season when a higher-ranking male became available (Otter andRatcliffe 1996, Ramsay et al. 2000). These examples highlight that variation in male availability and quality can influence female mate choice decisions even shortly before breeding onset. The availability of nesting sites is another important ecological factor known to fluctuate over time [START_REF] Newton | The role of nest sites in limiting the numbers of hole-nesting birds -a review[END_REF]) and known to lead to intense competition among males, females or pairs, whereby dominant individuals are expected to gain access to these resources [START_REF] Newton | The role of nest sites in limiting the numbers of hole-nesting birds -a review[END_REF]. A limitation of nesting sites may thus lead to a shortterm re-shuffling of breeding pairs (and hence within-season divorce) so that dominant males and females will manage to breed while sub-dominant individuals are evicted.

During an experimental field study, [START_REF] Jacot | Experimental nest site limitation affects reproductive strategies and parental investment in a hole-nesting passerine[END_REF] investigated the effects of limiting nesting sites on the mating and reproductive behaviour of blue tits. We observed that several breeding pairs divorced shortly before breeding and subsequently re-mated with another individual. The main aims of this paper are (1) to describe these incidences of within-season divorce shortly before egg laying and (2) to report on an analysis of the consequences of within-season divorce on reproductive performance.

Material and methods

We studied the effects of limiting nest sites on reproductive behaviour in a population of blue tits, Cyanistes caeruleus, breeding in nestboxes at Kolbeterberg (Vienna, Austria; 48°13'17"N, 16°14'12"E). For a detailed account of the nest-site limitation experiment see [START_REF] Jacot | Experimental nest site limitation affects reproductive strategies and parental investment in a hole-nesting passerine[END_REF]. In short, we reduced the number of nestboxes by 50% in two experimental plots a few days before egg laying started in the population (5 th of April), and kept two plots as controls. After the manipulation, the experimental plots contained 10 and 9 nestboxes, whereas the control plots had 22 and 17 boxes.

Most importantly, we removed all nestboxes (active and inactive) in the afternoon of the 5 th of April, and erected new nestboxes in the same location (control plots) or at new locations (experimental plots) in the following night. All breeding pairs in the study site thereby experienced similar levels of disturbance, i.e. their nests were removed, but pairs in the experimental plots experienced stronger competition for a new nest site than pairs in control plots.

During the pre-experimental period (mid March -5 th of April) each box was visited daily and the identity of the individuals showing territorial behaviour was recorded. Unbanded individuals were trapped with a mist-net trap in the vicinity of the nestbox and marked with a unique combination of three plastic colour bands (AC Hughes, Middlesex, UK) and a numbered metal ring (Vogelwarte Radolfzell, Radolfzell, Germany). If the individuals were already banded we recorded their colour bands. For 65% of the active nestboxes we identified or banded both individuals, in 26% of the boxes we identified only one individual and in the remaining 9% of boxes none of the individuals could be identified or captured. The number of individuals identified (none, one, or both) did not differ between the control and experimental plots ( 2 2 = 2.90, p = 0.23, n = 78).

After the experimental manipulation on the 5 th of April, all nestboxes were visited again on a daily basis and we recorded the identity of territorial individuals. Clutch size, hatching success and fledging success were determined by regular nestbox checks. The identity of the breeding pair was confirmed by catching feeding adults when chicks were around 10 days old.

Statistical analyses

All statistical analyses were performed with R2.8.0 (R Development Core Team 2008). The function lm was used to fit general linear models investigating the effects of mating status on clutch size and hatchling and fledgling number. Hatching and fledging success were analysed by performing generalized linear models (function glm) with a binomial error distribution correcting for overdispersion. When investigating the effects of mating status on complete brood failure (y/n), we performed a Fisher's Exact Test for count data.

Results

For 31 out of a total of 56 active nests, we identified the breeding adults before and after the nest site limitation experiment. Of these 31 active nests, 7 nests (23%) were owned by pairs mated with a new partner (3 in control plots, 4 in experimental plots; Fig. 1). Within-season divorce was equally common in experimental and in control plots (Fisher's Exact Test: odds ratio: 0.23, n = 31, P = 0.17).

Figure 1 illustrates the movement between original and new nestboxes of each individual involved in within-season divorce. Breeding pairs of the 7 cases of within-season divorce were composed of 11 individuals that had been recorded in the study area before the experimental manipulation (solid arrows in Fig. 1) and of 3 unbanded individuals, whose origin is unknown (broken arrows in Fig. 1).

These 11 known individuals originally came from 10 different nestboxes, i.e. in 1 original nestbox both individuals re-mated with a new partner in the study area (see Fig. 1). The abandoned partners in the remaining 9 original nestboxes could not be identified in the field or captured afterwards and it thus remains unknown whether these individuals dispersed, died or remained as floaters in the study site. In two cases of within-season divorce, individuals moved from an original box in a nest-site limited plot to a new box in a control plot. In both cases, females dispersed from their original territory to re-mate with a previous territory holder in control plots (see Fig. 1). In contrast, there was no case where an individual moved from a control plot and re-mated with an individual in a nest-site limited plot.

Males and females from faithful and divorced (within-season) pairs did not differ in body mass, tarsus and wing length (separate t-tests for males and females; all p>0.2). In addition, faithful pairs (n=24) and pairs that re-mated with a new partner (n=7) from both treatments did not differ in lay date (faithful: 7.38 ± 0.5 days after manipulation, divorced: 10.71 ± 3.1; t 29 = -1.17, P = 0.25), in clutch size (faithful: 11.4 ± 0.4, divorced: 11.2 ± 0.9; t 29 = 0.84, P = 0.41) and in the number of chicks that hatched (faithful: 9.8 ± 0.5, divorced: 8.3 ± 1.4; t 28 = 1.14, P = 0.27). In contrast, the number of fledglings differed significantly between the two groups (faithful: 8.7 ± 0.8, divorced: 4.7 ± 1.9; t 28 = 2.29, P = 0.030). Accordingly, fledging success (i.e., % of eggs that resulted in fledglings) was lower in pairs that switched their partner (faithful: 0.75 ± 0.06, divorced: 0.45 ± 0.17; t 28 = 2.24, P = 0.033).

This effect is partly due to complete brood failures, which tended to be more common in broods where mate switching had occurred (faithful: 2 out of 24 broods, divorced: 3 out of 7 broods; Fisher's Exact Test: odds ratio: 7.48, n = 31, P = 0.062). None of these brood failures were due to nest predation by greater spotted woodpeckers or aesculapian snakes, common nest predators in our study area. Note that of these 5 incidents of complete brood failures, three occurred in control plots (2 faithful, 1 divorced) and two in experimental plots (both divorced; see Fig 1). In one out of the seven cases of within-season divorce, the brood was deserted during the incubation stage.

Discussion

Here we describe in a wild population of blue tits several cases of within-season divorce shortly before the onset of breeding. 23% of pairs divorced and at least one of the partners re-mated with a new territory holder. Within-season divorce was associated with fitness costs, in that pairs that re-mated had a lower fledging success. However, due to the small sample size these results have to be dealt with caution and we refrained from testing classical hypotheses to explain between-season divorce [START_REF] Choudhury | Divorce in birds -a review of the hypotheses[END_REF]. We further discuss causes of within-season divorce and potential mechanisms responsible for the reduced breeding success.

Data about within-season divorce in blue tits are not available in the literature and we are not able to link our observations to the naturally occurring rate of this type of divorce. Between-season divorce rate is at around 50% in our blue tit population [START_REF] Valcu | Causes and consequences of breeding dispersal and divorce in a blue tit, Cyanistes caeruleus, population[END_REF]) and includes any change of partner between two breeding seasons irrespective of the timing of the partner change.

It is possible that many of the reported cases of divorce happen shortly before breeding and our results may reflect a natural rate of within-season divorce. Alternatively, the relatively high rate of within-season divorce in this study was due to the experimental disturbance at the original nestbox shortly before egg laying. All nestboxes in control and experimental plots were temporarily removed from their original site and re-erected the following morning. This disturbance during the nestbuilding stage may have weakened the pair bond where several pair members tried to ensure breeding in the current season by re-mating with a new mate. Females that lost their nestbox in experimental plots might have successfully out-competed other females throughout the entire study area. Indeed, in two cases, females from experimental plots re-mated with males from control plots (see Fig. 1), supporting the idea that floating females may evict territorial females from their territory. This hypothesis is in line with studies documenting intense female-female competition and aggression for available nesting sites shortly before egg laying [START_REF] Slagsvold | Polygyny in birds -the role of competition between females for male parental care[END_REF], particularly also in blue tits [START_REF] Kempenaers | Polygyny in the blue tit -intra-sexual and inter-sexual conflicts[END_REF].

Breeding pairs showing within-season divorce had lower reproductive success compared to faithful pairs. These results contrast to previous findings in black-capped chickadees, which demonstrate that females benefited from within-season divorce by re-mating with a male of higher quality (Otter andRatcliffe 1996, Ramsay et al. 2000). The reduced fledging success in our study was partly explained by total brood failures during the incubation or nestling phase, which were equally common in control (n=3; 2 faithful, 1 divorced) and experimental (n=2; 2 divorced) broods. More studies on the fitness consequences of within-season divorce are certainly needed since our results are based on a small sample size and additionally on the occurrence of complete brood failures, which may be due to chance events like the death of one pair member. Alternatively, the reduced breeding success is due to the nest-site limitation experiment that altered breeding conditions in the whole study area. Increased competitive interactions around nestboxes may continue during the incubation and nestling phase, assuming that evicted individuals remained within the study site. The extra investment in territorial defence may trade-off with brood care or survival and there might be additional direct negative effects of the presence of floaters on reproductive performance. In one experimental box, dead nestlings were found with open wounds on their head which most likely originated from attacks by adult conspecifics. This type of injuries has never been observed before in this study population. Alternatively, the rapid change of partner shortly before egg laying may have altered a male's perceived confidence in paternity and this might negatively affect paternal investment [START_REF] Whittingham | Confidence of paternity and male parental care[END_REF][START_REF] Sheldon | Relating paternity to paternal care[END_REF] and may partly explain the complete brood loss via brood desertion of the male in control and experimental plots.

In contrast to other studies on within-season divorce (Otter andRatcliffe 1996, Ramsay et al. 2000), we could not demonstrate any fitness benefits of a rapid change of partner before breeding onset. However, under sudden nest site limitation, evicted individuals may have only two alternatives: either initiating a fast change of breeding partner and thereby gain some reproductive success, or forego breeding for one year. Hence, within-season divorce may reflect a flexible strategy of short-lived species such as the blue tit trying to maximize the probability to reproduce in the current season. This flexibility in mate choice is partly in line with a study in European starlings, which demonstrates that females adopted a similar re-mating strategy once their partner disappeared or died shortly before breeding [START_REF] Pinxten | Extra-pair paternity and intraspecific brood parasitism in the european starling, Sturnus vulgaris -evidence from DNA fingerprinting[END_REF]). To assess the adaptive value of within-season divorce one would need to compare residual reproductive success of individuals that switched their partner shortly before egg laying versus individuals that forewent a breeding bout. Fitness consequences of divorce are complex , but our results suggest that divorcing shortly before the onset of breeding may incur higher fitness costs compared to divorce and re-mating during the nonbreeding season, highlighting the importance of the optimal timing for mate-switching.

Zusammenfassung

Figure 1

 1 Figure1Map of the study site indicating occupied territories after the experimental manipulation. In

Acknowledgements

We thank Kees van Oers, Cornelia Ebert, Kim Teltscher, Agnes Tuerk and Zeynel Arslangündogdu for help during the field work. In addition, we thank two anonymous reviewers for constructive comments on the manuscript. The study received financial support from the Max Planck Society.