Age-related variation in wing shape of migratory and sedentary Blackcaps Sylvia atricapilla

In many passerines, juveniles have shorter and more rounded wings than adul ts. Given that (1) long and pointed wings improve endurance in migratory fl ights, (2) shorter and rounded wings improve manoeuvrability, and (3) juven ile birds are more vulnerable to predators than adults, it has been hypothe sised that ontogenetic variation in wing shape results from a greater impor tance of predation avoidance relative to migration performance during the f irst year of life. If so, wing shape should not change with age in the abse nce of migration-related selection for longer and more pointed wings. We te st this by studying the variation with respect to age in wing length and wi ng pointedness of migratory and sedentary Blackcaps wintering in southern S pain. Migratory Blackcaps had longer and more pointed wings than sedentary Blackcaps. Juveniles had shorter wings than adults in migratory populations , but not in sedentary populations. The variation with age in wing pointedn ess was less pronounced, and was found in migratory females only. These dif ferences between the two traits could be related to a stronger selection fo r pointed wings than for longer wings with increasing distance of migration , and to an increased migratoriness of females in partially migratory Black cap populations. We hypothesise that, in migratory Blackcaps, a shorter and more rounded wing in juveniles could be selected for if the decrease in pr edation rate compensated for the somewhat greater costs of the first migrat ion attempt. On the other hand, there are no costs of migration in sedentar y Blackcaps, which hence maintain a similar wing shape, giving high manoeuv rability, both as juveniles and as adults.