Differential timing of passage of populations of migratory Blackcaps (Sylvia atricapilla) in Spain: evidence from flight-associated morphology and recoveries

Abstract

We examined the timing of passage of migratory Blackcaps (Sylvia atricapilld) at a stop-over site in northern Iberia, Spain, in an area relatively close to an important wintering area for the species, in southern Iberia. With this goal, we analysed ring-recovery data and examined the seasonal, year-to-year and age variations of flight-associated morphology (wing length and wing morphology). In autumn, Blackcaps with longer and more pointed wings (supposedly long-distance migrants) were captured significantly later than birds with shorter and less pointed wings (i.e., less pronounced long-distance morphology). Moreover, in autumn Blackcaps from the Britain and Ireland were proportionally more abundant than Blackcaps from mainland Europe during early autumn than during late autumn, supporting the hypothesis that Blackcaps from the Britain and Ireland migrate earlier than those from mainland Europe. These findings suggest that Blackcaps from the Britain and Ireland might have the wing morphology characteristics for shorter-distance rather than long-distance migration. In spring, however, wing length, wing morphology and the origin (Britain and Ireland versus mainland western Europe) did not vary with trapping date. This lack of differences in spring suggests an absence of a differential timing of passage by different Blackcap populations in spring. However, we may have been unable to detect such differences due to a small sample size or the fact that spring migration takes place within a shorter period than autumn migration.