In migratory birds, morphological adaptations for efficient migratory flight often oppose morphological adaptations for efficient behavior during resident periods. This includes adaptations in wing shape for either flying long distances or foraging in the vegetation and in climate-driven variation of body size. In addition, the timing of migratory flights and particularly the timely arrival at local breeding sites is crucial because fitness prospects depend on site-specific phenology. Thus, adaptations for efficient long-distance flights might be also related to conditions at destination areas. For an obligatory long-distance migrant, the common nightingale, we verified that wing length as the aerodynamically important trait, but not structural body size increased from the western to the eastern parts of the species range. In contrast with expectation from aerodynamic theory, however, wing length did not increase with increasing migration distances. Instead, wing length was associated with the phenology at breeding destinations, namely the speed of local spring green-up. We argue that longer wings are beneficial for adjusting migration speed to local conditions for birds breeding in habitats with fast spring green-up and thus short optimal arrival periods. We suggest that the speed of spring green-up at breeding sites is a fundamental variable determining the timing of migration that fine tune phenotypes in migrants across their range.

Department of Anatomy Cell and Developmental Biology Eötvös Loránd University Budapest Hungary

Department of Biology Ondokuz Mayis University Samsun Turkey

Department of Bird Migration Swiss Ornithological Institute Sempach Switzerland

Department of Zoology and Physical Anthropology Complutense University of Madrid Madrid Spain; SEO Monticola Ringing Group Autonomous University of Madrid Madrid Spain

Institute for Environmental Protection and Research Ozzano dell'Emilia Italy

Institute of Biodiversity and Ecosystem Research Bulgarian Academy of Sciences Sofia Bulgaria

Michael Otto Institut in NABU Bergenhusen Germany

Vilová 246 Kněžmost Czech Republic

Zoological Institute University of Basel Basel Switzerland

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