No. 493] 



CHANGES OF COLOR IN BIRDS 



35 



General condition of the bird's body— whether fat or 

 thin. 



Food— whether vegetable or animal. 

 Blood pressure— whether raised or lowered. 

 Sexual organs— whether active or inactive. 

 Inheritance. 



Temperature— heat or cold. 



Conditions of humidity or aridity. 



This list is of course merely tentative, and the factors 

 enumerated are by no means equal, and some are depend- 

 ent on others. But they represent what I selected when 

 first I began the experiment detailed below, as a con- 

 venient review of the field before me. 



This experiment concerns only factor number one, the 

 condition of fatness or thinness of the bird's body and 

 its influence on moult and indirectly on the sequence of 

 annual changes of color. 



We know that birds, such as bobolinks and tanagers, 

 after the cares of the breeding season are always thin 

 and in poor condition. The externally worn and bedrag- 

 gled condition of the feathers is reflected in the physically 

 deeper part of the body, and the keel of the breast-bone— 

 that true index of a bird's condition— often is very con- 

 spicuous under the skin of the breast. Not until the fall 

 moult is past do the birds improve much in condition and 

 then they become unusually fat, I think that these few 



Fat is something to guard against in captive birds, and 

 in the Zoological Park T find it necessary to have a 

 weekly examination made of many of the small birds; 

 this being a matter of regular routine. Birds from the 

 various cages are caught and carefully examined, and the 

 proportions of the food ingredients -fat-producing and 

 the opposite-are regulated according to the condition of 

 the birds. 



One vear ago last summer I took full-plumaged speci- 

 mens of male scarlet tanagers and bobolinks m full song 

 and plumage and put them under careful observation. 



