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

In many passerines, juveniles have shorter and more rounded wings than adults. Given that (1) long and pointed wings improve endurance in migratory flights, (2) shorter and rounded wings improve manoeuvrability, and (3) juvenile birds are more vulnerable to predators than adults, it has been hypothesised that ontogenetic variation in wing shape results from a greater importance of predation avoidance relative to migration performance during the first year of life. If so, wing shape should not change with age in the absence of migration-related selection for longer and more pointed wings. We test this by studying the variation with respect to age in wing length and wing pointedness of migratory and sedentary Blackcaps wintering in southern Spain. 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 pointedness was less pronounced, and was found in migratory females only. These differences between the two traits could be related to a stronger selection for pointed wings than for longer wings with increasing distance of migration, and to an increased migratoriness of females in partially migratory Blackcap populations. We hypothesise that, in migratory Blackcaps, a shorter and more rounded wing in juveniles could be selected for if the decrease in predation rate compensated for the somewhat greater costs of the first migration attempt. On the other hand, there are no costs of migration in sedentary Blackcaps, which hence maintain a similar wing shape, giving high manoeuvrability, both as juveniles and as adults.