Does urbanization affect wing pointedness in the Blackbird Turdus merula?

Abstract

Colonization of urban habitats requires physiological, behavioral and morphological changes in many species. This is well studied in the Common Blackbird (Turdus merula), one of the most common bird species in Europe. Former studies found that urban Blackbirds show decreased migratory behavior. However, it is still a matter of debate whether this is linked to morphological changes. According to Seebohm’s rule, sedentary individuals have less pointed wings in contrast to migratory individuals. Here we assess if wing pointedness—as measured by Kipp’s index—decreases along a rural to urban gradient in Blackbirds. To determine this, Blackbirds were caught in Frankfurt am Main and Heidelberg, as well as in the neighboring uplands of the Taunus and Odenwald, along a virtual continuous urbanization gradient, ranging from nearby forests to city centers. Along this gradient, wing pointedness significantly decreased in male, but not in female, Blackbirds. However, this trend was no longer significant when considering single regions. This indicates that wing pointedness does not change uniformly across the species’ range because of different intensities of urbanization. Sex-specific differences might be due to distinct territorial and dominance behavior. Variations in wing shape of urban and rural Blackbird populations are most likely based on a combination of better climatic conditions and increased food availability, as well as behavior adjusted to urban habitats. This study shows urbanization effects on the wing morphology of Blackbirds that might coincide with reduced migratory behavior.