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Short note: Willow Warbler Phylloscopus trochilus male moulting and suspending before feeding replacement brood |
| For readers from other continents: The Willow Warbler Phylloscopus trochilus is a North Palearctic species, breeding almost to the Pacific in Siberia. The brooding period is appr. 13 days, young are fed in the nest for at least two weeks. All populations have a full postnuptial moult, starting when young are still being fed in late June (south Sweden) or July (Lapland), males starting before females. |
On 23 June 2002 a male Willow Warbler was caught in my garden in Ljunghusen, 55º 24' N, 12º 55' E, SW Scania, Sweden. Primaries 1 and 2 had been exchanged in both wings and were full-grown, no other remiges were shed or growing; the bird obviously had started moulting and suspended. Under normal conditions (breeding finished and moult initiated) P3 and P4 - and even P5 - would be shed before P1 and P2 were full-grown. This particular bird had held a territory including the garden throughout May, and a nest-building/brooding female had been present from the second half of May. Then both birds suddenly were lost out of sight, and I assumed that the brood or either of the parent birds had been predated and that the surviving bird(s) had left the area. On 23 June, however, the male was collecting food and warning when I approached a certain area in the old territory. It is quite obvious, that it was feeding a replacement brood, and that it had moulted two primaries and then suspended while some female, the old one or a new, was brooding in early June. The female had not been seen collecting food up till and including 23 June, indicating that the young were newly hatched on that date. (On 30 June, 14h, the female was caught; she was unringed and had brood-patch with retarded blood-vessels, indicating that she was no longer brooding. P1 and P2 were shed, no pins were visible. The male sang close to the net but was able to evade it. Both Willow Warblers continued to collect food together throughout the afternoon; the day was windy, rainy and rather cold, newly fledged Pied Flycatchers were still begging at sunset, there was little or no food for them. The young fledged on 9 July and were moved away from the area on 10 July).
In many species there seems to be some degree of freedom relative to the necessity of exchange of feathers: the process can be manipulated. In South Sweden males from species with a complete postnuptial moult often start before females; in the present case the male made the most of a sudden delay in the breeding and exchanged two primaries. I have never been able to prove the presence of this strategy before, although I have suspected that it existed.
This note was written by Christer Persson, published on the web 23.6.02, addendum 30.6.02.