NO. 12 BODY TEMPERATURE OF BIRDS — VVETMORE 2/ 



of some species of birds to withstand bitter cold winter weather may 

 be better understood. 



Temperature control among birds is less perfect in juvenile than 

 in adult individuals so that the action described in the case of young 

 herons in the preceding paragraph is of great aid to control of heat 

 through the air-sacs. As temperature is poorly regulated in young 

 individuals parent birds often find it highly necessary to shelter their 

 offspring when these are reared in exposed nests. On hot cloudless 

 days, therefore, one bird of each pair remains constantly at the nest 

 during the warmer part of the day, and intervenes its body and partly 

 spread wings between the young in the nest and the burning rays of the 

 sun. I have seen many young herons and ibises perish when the 

 adults were driven from their nests on hot days, and during field work 

 in rookeries of these birds have made it a point to visit them during 

 the cooler portions of morning or afternoon, or to come on days 

 when the sky was overcast by clouds in order to prevent such mortality. 

 As another evidence of poorer temperature control in young birds I 

 may add that in several cases I have seen immature coots (Fulica 

 americana) die, apparently of sunstroke, when unduly excited while 

 exposed to the burning rays of a western sun. Adult birds seem able 

 to react against these circumstances in such a way that they do not 

 succumb but often exhibit evident signs of severe suffering. It is 

 probable that the more perfect development of the feathered covering 

 in adult birds is of as great advantage in this as the increased efficiency 

 of the heat regulatory organization in the body. 



On a few occasions I have observed a further development of the 

 function of temperature control by air-sacs in certain forms of birds 

 while in the fledgling state. Those who have had occasion to work 

 in summer in marshes densely grown with rushes will agree that at 

 times the heat encountered is almost overpowering. In a few instances 

 in such situations I have observed young yellow-headed blackbirds 

 recently from the nest, resting quietly with the cervical air-sacs 

 immensely swollen so that the lower part of the neck was greatly 

 enlarged. The whole gave the appearance of some unwholesome 

 tumorous growth and at first I was under the impression that the 

 birds were diseased. On handling them, however, the sacs rapidly 

 subsided and the birds seemed normal in every respect. The same 

 phenomenon has been observed in young savanna sparrows and in 

 young red-winged blackbirds. In these cases I was forced to conclude 

 that the distended air-sacs form an insulation or protection against 

 heat from without. In other words, that the enlarged cavity of 

 the sac acted as a dead air space protecting the blood stream in the 



