FACTORS AND STIMULI 213 



centage of traveling birds was somewhat unexpected, but it proved 

 less surprising than the results obtained from the remaining groups. 

 The behavior of the capons was wholly unforeseen. The control 

 capons instead of proving sedentary, all traveled southeast. The 

 experimental capons, although not quite so uniform, were also south- 

 ward-bound. These two groups seem to have settled one point. 

 "Whatever may be the case with the northward migration, the south- 

 ward is evidently not associated with the state of the reproductive 

 organs. The movement must depend on some other, at present unde- 

 termined factor. . . . That a revision of the original hypothesis is 

 necessary is evident. . . . Castration does not inhibit the southward 

 passage, which appears to be independent of the influence of the 

 gonads." 



Kendeigh (1934). Kendeigh has recently carried out a compre- 

 hensive and thoroughgoing investigation of the response of the house 

 wren {Troglodytes aedon Vieillot) and English sparrow {Passer 

 domesticus L.) to physical factors, which gains much through the 

 previous studies of the temperatures of birds by Baldwin and Kendeigh 

 (1932) . The factors specially considered are temperature, solar radia- 

 tion, humidity, precipitation, and wind, each of which is treated with 

 respect to distribution, migration, and abundance. Of these, tempera- 

 ture is the most significant, though it bears a necessary relation to 

 the quantity and duration of radiation. For adult passerines, a body 

 temperature below 38.9° C. (102° F.) is suboptimal, though one as 

 low as 23.9° C. (75° F.) may be withstood for a short time. The 

 interval between the highest normal body temperature, 44.6° C. 

 (112.3° F.), and the lethal, 46.1° C. (116° F.), is much narrower, 

 with the consequence that air values of 35° C. (95° F.) or higher 

 seem to have a more immediate and fatal effect than low air tempera- 

 tures. During spring and autumn, night time is the critical period 

 since the body temperature reaches a minimum then, as does the air 

 temperature also, and the birds are inactive and without food for 

 several hours. For the day, especially in midsummer, the critical 

 time in the tolerance of high temperature falls at the afternoon maxi- 

 mum for air and bird. 



As for the role of food in survival and migration, it was found 

 that starvation lowers the body temperature materially, individuals 

 of three species dying when the latter fell to 32.8° C. (91.1° F.), as 

 a result of lessened resistance. Emphasis is also placed upon the high 

 food intake needed to maintain normal metabolism, granivores con- 

 suming daily about 13 per cent of body weight in air-dry food, 90 

 per cent of which is digested, while for insectivorous species the 



