Complete Post-Juvenile Moult in First-Year Blackcaps: Proximate Causes and Adaptive Implications

Moult is one of the most energy-demanding events in the annual cycle of a migratory bird. As a consequence, moult patterns have evolved that separate moult from other costly life-cycle stages and that exclude unnecessary replacement of the feathers. We studied two Spanish blackcap Sylvia atricapilla populations with the aim of elucidating variation and flexibility of their postjuvenile moult pattern. In contrast to most juveniles of this species, which normally undergo a partial moult both in captivity and in the wild, we found complete moult in 2 juvenile blackcaps. Complete moult was also inferred for another 7 birds, which were in active moult. Eight of these birds were trapped at a site in northern Spain and one originated from a partially migratory population from eastern Spain. This bird, held under the same environmental conditions as other 31 juveniles from the same population, moulted all primaries in captivity while the other 31 juveniles invariably undertook a normal partial post-juvenile moult. Therefore, it seems likely that complete post-juvenile moult observed in this bird was not induced by environmental conditions but had a genetic basis. Thus, we show that previously undetected variation among post-juvenile moult patterns is present in wild blackcaps and suggest that variation in the expression of important life-history traits could represent a potential reservoir for adaptive changes. This finding is in line with the great adaptive capacity of blackcap, repeatedly observed both in wild and in captivity.