Investigating the influence of local weather conditions and alternative prey composition on the breeding performance of urban Eurasian Kestrels
Capsule: Local weather conditions, but not a city associated diet composition, influence the breeding performance of urban Eurasian Kestrels Falco tinnunculus. Aims: We aim to explore the impact of diet composition, specifically a high proportion of avian prey, on the breeding performance of urban Eurasian Kestrels under different weather conditions. Methods: The Eurasian Kestrel is known to exploit cities and occurs in the study area of Vienna, Austria (415 km) at high breeding densities of 89 122 pairs/100 km. Unlike their rural conspecifics which specialize in voles, urban Kestrels are generalists, preying on passerines and mammals, but also reptiles and insects. We explore this alternative diet through pellet analyses along an urbanization gradient over a 5 year period and link diet to reproductive performance, taking local weather conditions into account. Results: Our results showed that weather had a greater influence on breeding performance than did diet. Warm and dry weather during the arrival and courtship period was correlated with earlier egg laying, higher nestling survival and an overall higher breeding success. Dry winters increased the proportion of mammalian prey, whereas low temperatures and high rainfall during the nestling phase increased the proportion of avian prey. Overall, a more diverse diet was also associated with higher rainfall, but only during the arrival and courtship period. Conclusion: Our research shows a stronger connection between breeding performance and weather conditions than between breeding performance and diet composition, although there may be indirect effects of weather on prey availability which augment the impact of diet on reproductive output in the Eurasian Kestrel. ARTICLE HISTORY Received 14 January 2016 Accepted 24 May 2016 Urbanization is a global anthropogenic phenomenon which results in the increase of impervious surfaces, the loss of green space and high turnover of species composition (Ramalho Hobbs 2012) Among birds, urbanization can result in a population decline for many species (Sol et al. 2014) while others benefit from the altered conditions which may increase availability of food and nest sites (Rodewald Bakermans 2006, Gonzalez Oreja 2011) Blair (1996) categorized bird communities along urban gradients and defined 'urban avoiders' which are restricted to their native, undisturbed habitats, and on the contrary a small number of 'urban exploiters' which are often non native (Jokimaki Suhonen 1998) and very successful in urbanized, human dominated environments. This division within avian communities is defined as 'biotic homogenization' a phenomenon linked to global urbanization processes (Blair 2001, McKinney 2006) More than 25 avian raptor species have been recorded worldwide in urban settings (Love Bird 2000) High food availability within urban settings can lead to positive population responses in predatory birds (Chace Walsh 2006) In particular, specialized bird feeders like the Goshawk Accipiter gentilis, the Sparrowhawk Accipiter nisus or some falcon species respond well to urban landscapes because of the large biomass of avian prey (Newton 1980, Cade et al. 1996, Berry et al. 1998, Kenward 2006, Suri 2016) Some raptor species extend their home ranges beyond urban boundaries (Chace Walsh 2006) and breed in inner city districts while hunting in rural areas (Riegert et al. 2007) (c) 2016 British Trust for Ornithology CONTACT P. Sumasgutner petra.sumasgutner@univie.ac.at Department of Integrative Zoology, University of Vienna, Althanstrasse 14, A 1090 Vienna, Austria Supplemental data for this article can be accessed here: http:/dx.doi.org/10.1080/00063657.2016.1213791. BIRD STUDY, 2016 http:/dx.doi.org/10.1080/00063657.2016.1213791 D ow nl oa de d by T ur ku U ni ve rs ity at 0 1: 57 0 5 A ug us t 2 01 6 This strategy might be limited to smaller cities and appears difficult in larger urbanized areas (Sumasgutner et al. 2014a) The Eurasian Kestrel Falco tinnunculus (hereafter Kestrel) is known to have inhabited and bred in urban environments since at least the second half of the 19th century (Cramp Tomlins 1966) Studies on this species have been conducted in several European cities, for example Bratislava (Darolova 1992) Warsaw (Rejt 2001) Rome (Piattella et al. 1999) and several cities in Israel (Charter et al. 2007) Drawing general conclusions on the status of urban Kestrels remains difficult, as each city presents unique qualities in terms of size (which affects distances to potential rural hunting grounds) building structure (abundance of suitable nest sites) the composition of vegetation and amount of urban green space (which influences availability of prey) In Vienna, Austria, the population density of Kestrels is especially high not only compared to other European cities but also when compared to rural areas (Gamauf 1991, Kostrzewa Kostrzewa 1993, Sumasgutner et al. 2014a) Because of the Kestrels' inability to construct nesting platforms, building cavities provide attractive breeding sites (Village 1990) In particular, roof openings, which are characteristic of Vienna's historical city centre, are well sheltered and are therefore preferred over open nest sites (Sumasgutner et al. 2014b) Kestrels are not true 'urban exploiters' though apart from using artificial breeding sites, they do not profit from other resources related to urban environments. On the contrary, their breeding success, measured through lower hatching rates and fledged brood sizes, actually decreases with increasing urbanization towards the city centre (Sumasgutner et al. 2014a) Similar observations were reported for passerine birds in urban environments (Chamberlain et al. 2009) To settle successfully in an urban habitat, an appropriate food supply is one of the most important requirements (Witt 2000) While their rural conspecifics specialize in hunting small rodents, especially voles, urban Kestrels are generalists, preying not only on mammals, but also largely on birds (Galanos 1991, Piattella et al. 1999, Salvati et al. 1999, Kubler et al. 2005, Sumasgutner et al. 2013) and even on reptiles and insects (Riegert et al. 2009, Sumasgutner et al. 2013) The amount of avian prey, mainly passerines, increases with increasing urban cover and can even become the predominant prey in city centres (Kubler et al. 2005, Sumasgutner et al. 2013, Sumasgutner et al. 2014b) Most rodent species inhabiting Vienna are nocturnal (Mitter et al. 2015) and therefore inaccessible to a diurnal raptor like the Kestrel. A rodent survey in Vienna in 2010 showed almost exclusively field mice of the genus Apodemus (98.4% of trapped species were Wood Mouse Apodemus sylvaticus, Yellow necked Mouse Apodemus flavicollis and Pygmy Field Mouse Apodemus uralensis) as main potential prey available within urban parks (Sumasgutner et al. 2014a) However the distances to suburban areas, where these diurnal rodents would be available, are long for inner city breeding pairs. It therefore seems more efficient for urban Kestrels to hunt within a smaller home range (Sumasgutner et al. 2014a) and shift to alternative prey available in closer proximity to the nest site. Besides the impact of the urbanization gradient on diet, there might also be a strong connection between diet composition and weather conditions, both of which influence the breeding phenology of a predatory bird. For example, in Fenno Scandinavia, prey availability follows a distinct 3 4 year vole cycle (Hansson Henttonen 1985, Korpimaki et al. 2005) strongly affecting Kestrels' breeding performance. During phases of high vole abundance, the egg laying starts earlier, clutch sizes are larger and the number of fledged offspring is higher than in low vole years (Korpimaki Wiehn 1998, Valkama et al. 2002) Due to fluctuating food conditions, raptors might be forced to switch to alternative prey items. This might affect specialist predators more than generalist predators due to their different foraging efficiencies (Terraube et al. 2011) This might also be true on an individual level. A shift in diet composition is one of the most pronounced characteristics of urban Kestrels, and inner city Kestrels are considered to be generalists compared to their rural counterparts. In spite of their broad diets, overall foraging efficiency was hypothesized to be low for Viennese Kestrels and starvation of chicks was named among the main causes for the observed decreased breeding success in innercity pairs (Sumasgutner et al. 2014a) On the other hand, a more heterogeneous habitat, such as a patchy urban environment, might also have the potential to buffer adverse weather conditions by providing a more diverse diet through the higher availability of alternative prey species. A wider diet breadth or alternative diet composition might positively affect the health status of the nestlings (see, e.g. Sternalski et al. 2012, Hegemann et al. 2013) Diet composition is therefore an important determinant of success for an urban breeding raptor, and it may be strongly linked with local weather conditions. This study aimed to identify the relationship between reproductive success of Kestrels and (i) an alternative diet composition associated with urbanization, (ii) local 2 A. KREIDERITS ET AL. D ow nl oa de d by T ur ku U ni ve rs ity at 0 1: 57 0 5 A ug us t 2 01 6 weather conditions and (iii) nest site features (i.e. location along the urban gradient and nest type) over a 5 year period (2010 14) We expect to find a link between the altered conditions in the urban habitat and Kestrels' breeding performance (Sumasgutner et al. 2014a, 2014b) and predict that the proportion of avian prey increases from the periphery to the city centre while productivity decreases. At the same time we predict a strong connection between diet composition and local weather conditions. We expect inner city birds to perform better during advers