incubation period of our living bird species, especially since 

 the opportunities for such study will grow, as time passes, 

 fewer and fewer before the devastating onrush of civil- 

 ization. 



Some of these conditions as outlined above may never 

 occur in nature, jet it is wise to bear them in mind, and to 

 eliminate their effects when engaged in studying the incu- 

 bation of any bird. 



It seems clear that a definite and correct explanation 

 of the factor or condition which determine the true length 

 of incubation would long since have been reached, had the 

 data been more abundant and more amenable to study and 

 analysis. 



Because of the several distorting influences outlined 

 above, there have arisen a number of explanations as to 

 what determines or fixes the length of the incubation period, 

 explanations which are now in order for detailed considera- 

 tion. 



Size of Bird 



Before reviewing the information bearing on this ex- 

 planation of the controlling factor of incubation duration, 

 it becomes necessary to define "size of bird." An examina- 

 tion of the different uses of this expression by various 

 authors sheds no light on exactly what it means with them, 

 and also gives no indication that all such writers mean by 

 it the same thing. One finds the following expressions in 

 current use: "(length of incubation) in a general way is 

 proportionate to the bird's size" (15) ; "(length of incuba- 

 tion) varies with the size and vitality of the bird" (12) ; 

 "duration of incubation in general depends on height* of 

 bird" (38) ; "according to the size of the bird the incuba- 

 tion period varies, short or long, with hummingbirds ten 

 days, with ostrich fifty days" (9). "Size of bird" may mean 

 its dimensions, or its bulk, or, by implication, its weight. 

 It does not seem possible that past writers on this subject 

 could have meant size as indicated by the usual linear meas- 

 urements given in describing a bird, for these would lead 

 into a maze of characters, i. e., length of neck, total length, 

 length of bill, tail, legs or body, or the standing height, 

 which are extremely plastic and subject to such wide varia- 

 tion as to make it inconceivable that such shifting characters 

 could directly influence so deep-rooted and inelastic a char- 

 acter as the duration of incubation; in other words, that 

 these variable characters could be paralleled by variations 

 in the almost changeless true length of incubation. 



As for bulk in a bird, one must first recall that it and 

 the usual measurements do not necessarily go hand in hand. 



•Meaning its stature. 



1? 



