26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



(Cathartidae) that like the screamers spend hours in soaring high 

 in air. 



The action of the air-sacs in controlHng the body temperature may- 

 be demonstrated clearly by the following. A house sparrow {Passer 

 domesticus) was caught across the neck in a spring mouse trap of the 

 " Out o' Sight " type. The bird struggled and fluttered violently 

 for approximately 60 seconds while at the same time ingress or egress 

 of air to or from the lungs was prevented by compression of the 

 trachea against the edge of the trap platform by the spring of the 

 trap. I reached this bird as it became quiet and found that the body 

 temperature (interthoracic) registered 114° Fahr. The violent, sus- 

 tained muscular exertion had produced a considerable amount of heat 

 that could not be given off as the accustomed outlet v/as blocked. The 

 temperature, therefore, rose several degrees above the usual maximum 

 for this species. These same factors operate occasionally when the 

 trachea of a wounded bird is clogged with blood that prevents the 

 passage of air. The air current must be cut off quickly, however, as 

 the temperature falls rapidly in a wounded bird even when it is 

 struggling. 



During hot weather it is common to see birds breathing rapidly 

 with the mouth held open. This facilitates the rapid inspiration and 

 expiration of air from the lungs. Cooling of the mucous membranes 

 of the posterior portion of the mouth may also be of slight aid in 

 reducing the excess internal heat. In the case of some young birds, 

 as, for example, young herons, there is in connection with this habit 

 of breathing with open mouth another development to aid in regulating 

 the internal temperature of the body. When overheated these birds 

 open the mouth widely so as to expand the capacious mouth cavity 

 and pharynx while at the same time the skin on the sides of the upper 

 throat is vibrated with great rapidity. The inner walls of the pharynx 

 and upper throat in the birds in question are highly vascular so that 

 the currents of air set in motion aid in cooling the blood exposed in 

 the radiating blood vessels found near the surface in the moist mucous 

 lining. Conversely it may well transpire that the checking of the 

 rapidity of interchange of air between the branches of the bronchi 

 and the air-sacs during extremely cold weather may bring about a 

 storage or an increase in internal heat. In other words the heat of the 

 body cavity may be held at a higher level by the cessation of inhala- 

 tion of constant supplies of cold air into the air-sacs. We may imagine 

 a delicate adjustment here that will vary expiration of heated air 

 at need. With the air-sacs acting thus as heat reservoirs the ability 



