Morphometric Variation of Five Iberian Blackcap Sylvia atricapilla Populations

Abstract

This paper analyses the variation of several morphological traits in five populations of Blackcaps Sylvia atricapilla distributed along a latitudinal gradient in the Iberian peninsula. The northern and central populations differ from the southern ones in their longer and more pointed wings, narrower bills, shorter tarsi and smaller body size. These features define two morphological groups and correlate with differences in their migration and feeding habits. Birds from northern and central Iberia breed in habitats with harsh winter conditions, which they abandon in autumn when they migrate to their wintering grounds. Birds from the mild, southern sectors remain there throughout the winter. Their migratory behaviour, and a stronger specialisation for feeding on foliage invertebrates, could explain the morphological differentiation of northern Blackcaps relative to southern ones. Our results suggest that the Iberian migratory populations might have descended from ancestral, southern-like ones, that have become adapted to exploit their seasonal breeding grounds.