NO. 12 BODY TEMPERATURE OF BIRDS WETMORE II 



holds true of the avocet {Kecurvirostra amcricana) where 14 males 

 averaged 106.6° and 12 females only 104.9°. ^^ good series of read- 

 ings for the black-necked stilt {Hinianfopus mcxicanus) showed the 

 same average (105.8°) for both males and females. Through the 

 family Scolopacidae the general average on the whole showed a 

 balance in favor of the males save in a few instances. Among the 

 Charadriidae two species, the black-bellied plover {Squatarola sqiiata- 

 rola) and the killdeer {Oxyechus vociferus) show a balance in favor 

 of the female, while in the snowy plover (AEgialitis nivosa) and the 

 mountain plover (Podasocys montanus) the reverse is true. In some 

 passerines the two sexes average about the same in degree of bodily 

 heat. Thus in the yellow-headed blackbird {Xanthocephalus xantho- 

 cephalus) males (29 records) averaged 108.3° &"d females (18 

 records) 108.2°. Similarly in the house finch (Corpodacus mexicanus) 

 males (20 records) averaged 108.9° ^.nd females (7 records) 108.8°. 

 In these species we have, therefore practically an agreement in both 

 sexes as the difiference noted, amounting to only one-tenth of one 

 degree may well result from accident in securing the records. 



It appears therefore that in many species of birds temperatures of 

 females are higher than those of males. In a considerable number, 

 however, the two sexes average about the same, though with more 

 information it may be found that there is a slight difiference in favor 

 of the females. In a few cases there is found a higher temperature 

 in males than in females. This last is true in the Ardeidae, the Pha- 

 laropodidae, the Scolopacidae and in part in the Charadriidae. 



In the average run of species of homoiothermal animals the differ- 

 ence in temperature between male and female may be ascribed to the 

 needs of sexual activity and reproduction. At least this higher tem- 

 perature seems correlated with certain phases of reproduction. In 

 many birds the care of the young devolves upon the female and she 

 has the higher body temperature. In the Phalaropodidae, on the con- 

 trary, it has long been known that the duties of incubation and the 

 rearing of the young fall to the lot of the male, so that in connection 

 with this it would appear that he has developed a higher body tem- 

 perature than the female. In the case of the Scolopacidae the writer 

 is prepared to state from his own observations that incubation and 

 brooding of the young may fall largely upon the male in the willet 

 (Catoptrophonis inoniafus) and it is suspected (though not yet 

 proven) that this may be true of the majority of the species in this 

 family. Males of this group examined in breeding season often 

 exhibit areas upon the lower surface of the body bare of feathers 

 where the skin is thickened and vascular as in birds that have been 



