42 The Bird 
37). This is also the winter plumage of the adult birds, 
both sexes moulting alike into the rich-hued breeding 
plumage (Fig. 38) of yellow, chocolate, and silver-gray. 
The feathers of the entire bird are moulted or fall out 
naturally at least once a year, and in some cases twice 
or even three times. If we were asked at what season the 
7 
Fic. 26.—Flight-feathers of Chimney Swift clogged with soot, showing necessity 
for moulting. 
principal annual moult would be most likely to occur, 
the fall of the year would suggest itself, and such is the 
case, for a number of good reasons. 
First, the hardest work which birds have to do, hatch- 
ing and caring for their young, has, at this season of the 
year, just been accomplished, and has doubtless told 
heavily on their plumage. Breast-feathers are worn thin, 
tails are badly frayed, and wing-pinions are broken and 
ragged. Two alternatives confront birds at this period. 
Those species which are to take their migratory flight 
