l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ^2 



cool to the touch than otherwise in spite of the high body heat. In 

 fact they are seldom warm save in the hottest weather. Frozen feet 

 and toes are not uncommon among domestic fowls but are seldom 

 encountered in wild birds. The skin of the feet and tarsi is smooth 

 and oily, so that in the case of aquatic species' water does not adhere 

 when the extremities are exposed to the air, thus preventing danger 

 through the formation of ice. Aquatic birds at rest frequently draw 

 up the feet one at a time beneath the long feathers covering the flanks, 

 and ducks often rest on frozen ground with both feet drawn up in this 

 manner, so that relief is available when needed. 



In some groups of birds the tarsi and at times the toes are well 

 protected by a covering of feathers that prevent the radiation of 

 heat. Such a development is found in ptarmigan and certain other 

 grouse, in our owls of northern habitat, and in the sand grouse and 

 rough-legged hawks. A similar covering, though less dense and 

 heavy, is found,, however, in other birds that never encounter severe 

 cold. Thus the tarsus is feathered in whole or in part in some trogons, 

 in whippoorwills, in certain species of edible-nest swiftlets {Collo- 

 calia) and many others. The entire tarsi and upper surfaces of the 

 toes are feathered in the Old World martin (Delichon urbica). Al- 

 though this covering is present in Delichon, in the bank swallow 

 (Riparia riparia) a species that also breeds regularly far north in 

 Arctic regions, the feathering is restricted to a small tuft on the 

 posterior face of the tarsus near its lower end. Where this covering 

 of the legs and feet is found in species that at present do not seem 

 to require it for protection it may be supposed that it has persisted 

 after an ancient need causing the growth has disappeared, or that 

 it has developed as a correlated structure, perhaps ornamental in 

 nature. Thus in tropical owls the feathering of tarsi and toes is 

 greatly reduced, although in northern species it is very heavy. 



On the whole it would appear that radiation of heat through the 

 lower extremities is comparatively slight. 



DIVERSE MISCELLANEOUS FACTORS IN THEIR RELATION TO 

 BODY TEMPERATURE 



Previous sections have covered various phases of variations in 

 temperature due to sex, daily temperature rhythm and other condi- 

 tions. It remains to consider a few miscellaneous factors that afifect 

 this matter. Some of these are normal and some abnormal. 



The ingestion of large masses of food will frequently cause a 

 sudden decrease in body temperature in a bird of small size. The 

 matter swallowed if cold will absorb warmth until it has acquired a 



