the length of the incubation period, though one or two gen- 

 eral statements have been encountered, given, however, with- 

 out any detailed facts in support of the same. Newton (25) 

 says that there have been "no observations made on the ques- 

 tion if there be a difference in the length of incubation be- 

 tween polar and tropical individuals of the same species." 

 Arrigoni (12) is the only writer whose statement on this point 

 is definite, though it is unaccompanied by substantiating 

 facts: he says "it seems, however, that birds of the same 

 species which nest near the pole, and near the tropics, have 

 equal periods of incubation" (sembra pero che gli nidificono 

 e presso al polo e presso i tropici abbiano equale periodo de 

 incubazione). It is highly probable that the bodily temper- 

 atures of birds (as is the case with Homo) vary little, if at 

 all, with changes of zone, being, most likely, the same at the 

 polar region as at the tropics; there is, however, some evi- 

 dence which points to the possibility that atmospheric con- 

 ditions may influence a bird's temperature, but the question 

 has not yet been thoroughly studied and worked out. Never- 

 theless, I believe that birds, while incubating, would be able 

 to maintain the necessary optimum incubating temperature 

 equally well in the cold zones as in the hot zones. In this 

 connection, one must not forget that a large number of birds' 

 nests are constructed of such material and in such a manner, 

 as to retain to the best advantage the heat applied by the 

 parents to the eggs, facts which bear especially on the 

 question of incubation at the polar regions, the eiders being 

 marked examples of birds with nests built to retain heat. 

 The huge, bulky magpie nest has a central bowl of non- 

 conducting clay or mud, and in many nests are incorporated 

 most excellent non-conductors of vegetable or animal matter. 

 The chipping sparrow uses horse-hair, the writer has found 

 a nest of the Arkansas kingbird lined with rabbit's hair and 

 a house finch's nest lined with sheep-wool; and what better 

 non-conductor can be found than the cotton-like mass of 

 the hummingbird's nest? 



Far be it from the writer wishing to be understood as 

 holding that these materials are consciously selected for the 

 purpose by the nest-builders; attention is merely called to 

 the fact that provision to retain the applied heat is to be 

 found in many differing nests, and the nests of the polar 

 regions are no exceptions. 



Many birds successfully go through the duties of incu- 

 bation during the winter, when the forests are deep with 

 snow, and akin to the arctic regions, yet, so far as I know, 

 this does not seem to change their incubation period in com- 

 parison with their relatives nesting under less rigorous con- 

 ditions. If the foregoing be true, it seems to me that geo- 

 graphical zone has no effect on the true length of incuba- 



40 



