In England juvenile Willow warblers undergo premigratory lipid deposition in August. Prior to this there is a moult of the juvenile plumage which results in a reduction in both the glycogen content and the wet weight of the pectoralis muscles, the latter reflecting a decrease in the lipid-free dry weight. Muscle lipid levels are apparently unaffected by this moult. The decrease in the weight of the lipid-free dry fraction of the body extends also to components other than these muscles at this time. A correlation between the wet weight and the glycogen content of the pectoralis muscles suggests that cold stress may be the cause of the low glycogen levels found in the middle of the moult; part of this increased demand for thermogenesis may be due to lower body insulation brought about by feather loss during the moult. The muscle “hypertrophy” found in the premigratory period is simply a return to pre-moult weights.
The uniformity of total body weights during and after the moult indicates that at the end of this moult the water fraction of the body decreases in weight. The pectoralis muscles of juvenile Willow warblers are not fully grown on fledging; these unmoulted birds may also have higher body lipid levels than during the following moult.