Friday, 28 September 2007

Woodpeckers: juvenile moult

Juvenile woodpeckers begin a partial post juvenile moult at around the time of fledging and this continues until the autumn. In this first moult, juveniles of all ten European woodpecker species replace their primaries, rectrices and overall (body) plumage but some tertials, secondaries, primary coverts and outer greater coverts are retained. One of the ways to age 1st winter birds is the fact that there is often a moult limit in the greater coverts. For example, in Great Spotted Woodpecker where the new adult-type black inner greater coverts, contrast with the old juvenile brownish outer greater coverts. Juveniles have shorter tails than adults, due to all but the central pair of rectrices being shorter than on adults. The odd looking stunted tail of juvenile Black Woodpeckers in flight is due to this tail arrangement. Another interesting case involves Three-toed Woodpecker. Juveniles of this species sometimes begin their moult while still in the nesting cavity. Interestingly, the fifth to seventh innermost primaries are grown and replaced before the birds have fledged and thus they have never been used. This apparent rush to moult is almost certainly an adaptive feature, the result of the need of the species, as a bird of mainly northern latitudes, to complete the moulting task before the end of the short summer season. When the harsh winter season begins birds must be ready to disperse at short notice to new foraging areas, often some distance away, and a delay while moult is completed could prove fatal. For similar reasons, suspended moult occurs in local populations of other species that are prone to eruptive movements such as Fenno-Scandic and Russian Great Spotted Woodpeckers. Juveniles trapped in Fenno-Scandia after arriving from further east in late autumn apparently often exhibit signs of suspended moult in the tail and wing when compared to resident birds. Moult is then resumed sometimes as late as December.


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