254 . Zoologica : N. Y. Zoological Society. [I ; 14 



and pattern. These again may operate directly ontogenetically, 

 or indirectly along phylogenetic lines. 



Condition of the bird's body — fat or thin. 



Food — amount, and whether vegetable or animal. 



Blood pressure — raised or lowered. 



Sexual organs — active or inactive. 



Inheritance. 



Temperature — heat or cold. 



Meterological conditions of humidity or aridity. 



These are only the most apparent factors, they are of un- 

 equal value, and some are almost wholly dependent upon others. 

 •They represent what I tentatively selected when I first began 

 these experiments, as a convenient review of the general field. 



This experiment concerns only the first of the above factors, 

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

 influence on moult. I do not claim that it has further influence 

 than this. The unexpected result in color change or lack of it, 

 must be concerned with influences, other than immediate, acute, 

 physiological conditions. 



After all the stress and cares of the breeding season are 

 past, birds such as tanagers and bobolinks are always thin and 

 in poor condition. The worn and bedraggled feathers reflect 

 the actual physical state. The keel of the breast-bone — ^that true 

 index of a bird's emaciation or obesity — often protrudes conspic- 

 uously beneath the skin. Not until the autumn moult is com- 

 plete do the birds begin to improve and then they may become 

 unusually fat. These general facts doubtless hold good in the 

 case of most birds. 



Fat is one of the most insidious dangers incident upon a 

 collection of living birds. Unlike the condition in mammals 

 there is liitle or no external evidence of increasing obesity, and 

 only when the bird is in the hand and the breast feathers blown 

 aside, are the yellow rolls of adipose tissue visible. In the Zoo- 

 logical Park it is necessary to examine many small birds at fre- 

 quent intervals to ascertain their condition, and to regulate the 

 proportions of the food ingredients accordingly. 



