32 THE BIOLOGY OF BIRDS 



Perhaps the most important experiments on mouhing 

 that have yet been made are those of C. W. Beebe (1914) 

 on males of the Scarlet Tanager {Pira?iga erythromelas) and 

 the Bobolink {Dolichonyx oryzwonis), in both of which a 

 brilliant summer plumage alternates with a totally different 

 winter garb. The factor investigated was the condition of 

 fatness or thinness of the bird's body. 



Tame tanagers and bobolinks which had not bred but 

 were still at the height of vocal and physical condition were 

 placed at midsummer in small cages in a quiet room. The 

 light was gradually cut down and the food supply was 

 slightly increased. The birds became quiet and inactive ; 

 they rapidly put on fat and increased in weight. The 

 autumn moulting time passed without a single feather 

 being shed. Midwinter found the birds in their summer 

 dress, and it was noted that if one was temporarily brought 

 into a stronger light and had meal-worms added to its diet, 

 the song was fully resumed for a limited period. A sudden 

 alteration in temperature, up or down, reduced the weight, 

 and one of the tanagers, rapidly reduced, underwent a 

 belated moult into the green winter plumage. 



" Early in the following spring individual tanagers and 

 bobolinks [from among those experimented on] were 

 gradually brought under normal conditions and into their 

 seasonal activities, with quick result." The spring moult 

 took place, and the birds went directly from one nuptial 

 plumage to another. " The old scarlet and black feathers 

 fell from the tanagers, and were replaced by others of the 

 same colour ; and from buff, cream, and black, the bobolinlis 

 moulted into buff, cream, and black ! " In every case the 

 intervening winter plumage had been entirely suppressed. 

 It may be noted that the green winter plumage of the male 

 Scarlet Tanager is the permanent dress of the young of 

 both sexes and of the adult female. It is presumably 

 the ancestral garb. 



Mr. Beebe 's conclusion was that the condition of the 

 birds as regards fatness or thinness determines whether 

 they shall moult or not. I'he seasonal " pigmental changes 



