Decoupling of behavioural and morphological differentiation in a partially migratory bird population

Abstract:
Capsule Resident Blackcaps in eastern Spain differ in morphology and isotope profile from central European conspecifics wintering in the area, but not from locally breeding migrants. Aims Natural selection shapes flight morphology in relation to migratory habits. Thus, intraspecific variation in migratory behaviour is often paralleled by morphological differentiation. We test whether differences in flight morphology between migrants and residents can evolve in a partially migratory bird population. Methods We explored morphological differences among three categories of Blackcaps Sylvia atricapilla from a single study site in eastern Spain: residents, locally breeding migrants and wintering birds. We used discriminant function analyses to determine whether a set of morphological features was effective in predicting migratory status. In addition, we analysed deuterium (delta D) in claws and feathers to identify wintering areas. Results Discriminant functions identified central European Blackcaps wintering in the area, but failed to find morphological differences between residents and local migrants that also did not differ in isotopic profiles. Conclusions Lack of morphological and isotopic differences between migrant and resident fractions of the partially migratory population, suggests a facultative determination of migratory behaviour. Alternatively, migratory movements of migrants from this population may be too short to be detected using stable-isotope signatures.