Post-nuptial moult variation in the Willow Warbler Phylloscopus trochilus in relation to breeding

Abstract

In a nine-year study of post-nuptial moult interruption in the Willow Warbler, three moult patterns were noted which differed from the normal pattern. All concerned late-breeding individuals. One pattern was to suspend moult after the first two inner primaries were renewed and recommence at the point of suspension when young had attained independence. Another pattern was a very slow renewal of inner primaries with adjacent feathers only dropped when previous ones were fully grown and subsequently proceeding into a full moult when young were independent. In the third pattern, birds commenced their moult at an earlier stage of the breeding cycle, i.e. before young were independent. In all three patterns, post-nuptial moult would be completed earlier than if all birds waited until young achieved independence before moult was initiated, as is usually the case in adults with normal-timed broods.