unable to state ; nevertheless, it seems important in artificial 

 conditions, since most poultry raisers insist that with hens' 

 eggs in artificial incubation it is very essential. The second 

 is obviously a sine qua non, yet being largely one of physics, 

 it does not seem in reality to affect the length of incubation, 

 but rather the life or death of the embryo. Hence, since 

 neither of the first two essentials for successful incubation 

 is one of a bird's physiology, it remains to take up the last, 

 and since the heat applied to eggs in natural incubation 

 emanates from the setting bird, it would appear that the 

 factor of the degree of temperature applied to the eggs is 

 one of pure physiology, a question of the production and 

 application of animal heat. Moreover, it can be said, with- 

 out fear of contradiction, that temperature is the most im- 

 portant of all the three factors (or conditions) just enumer- 

 ated. Now, inasmuch as the incubation heat comes from 

 the brooding parent (true or foster), excepting with the 

 megapods and (possibly for a part of daylight hours) with 

 ostriches and the Egyptian plover, it would appear that the 

 bodily temperature of .the incubation parent should be a 

 highly important factor in relation to the incubation period. 

 In brief, it seems to me that birds' temperatures should be 

 investigated, not only as such, but also as to any relation 

 they may bear to the incubation period, and also any other 

 facts cognate with birds' temperatures. 



If a bird's temperature be highly necessary to successful 

 incubation, it would seem reasonable to predict that birds 

 have acquired habits, and conditions of body exhibited only 

 during incubation, which are calculated to facilitate the 

 application of, and conserve, the heat applied to the eggs 

 during this period. 



A superficial consideration of birds' nests throws some 

 light on this question. While a goodly number of nests are 

 too flimsy, or in too close contact, apparently, with the earth 

 to aid in concentrating the heat applied to the eggs, never- 

 theless a majority of nests are so constructed as to retain 

 this heat most advantageously. It is obvious, however, that 

 nests may have been developed, and probably did arise, as 

 a protection to, and receptacle for, the eggs, yet the almost 

 innumerable instances where the nest materials are ideal as 

 insulating media show that out of this primary use of a nest 

 has grown the concomitant result of the conservation of the 

 all-important heat applied during incubation. 



Reciprocally there are conditions normal to the bird, or 

 found only during the incubation time, which lend them- 

 selves to the application of a maximum amount of the par- 

 ent's heat to its egg, and to holding this heat at the most 

 advantageous level. Lucas (10) has maintained that the 

 majority of birds have no feathers on the abdominal area 



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