of a full set of eggs is laid, or only after all of a full set 

 is laid. 



I believe that the evidence permits one to hold that the 

 true length of incubation varies little, if at all, with the 

 species, however widely separated geographically, a view 

 substantiated by the Old and New World cuckoos, and the 

 small finches of the Northern Hemisphere and Australia. 

 However, the work of Cole and Kirkpatrick (92) seems to 

 show that at least with pigeons there may be a distinct, 

 though small, true variability in the length of incubation of 

 these birds. Whether this be a true variability, or one due 

 to retardation of development, is unknown to me ; an answer 

 to this point must probably be reserved until further light 

 is shed by future studies on the true lengths of incubation 

 in other species, and the possibility of such lengths being 

 really variable. The length of the incubation period must 

 be measured from the time the parent (or its substitute) 

 begins the steady application of heat to the eggs until the 

 young bird is fully released from its shell. 



The Data 



This study is based largely on the list of incubation 

 period lengths incorporated in this book. See Table No. 1, 

 which is made chiefly from lists published previously by 

 Evans (1 and 2) and by Bums (3), together with records 

 published singly by many others, plus those determined 

 and given to the writer by obliging friends. 



The incubation length data include records from 625 

 species and sub-species, scattered amongst 84 families, and 

 representing every order of existing bird., The writer has 

 tried to give each record in the words of the original con- 

 tributor or compiler*, and a list of all reference is given in 

 the bibliography. 



It is necessary here to note that some, perhaps many, 

 of these records may be duplicates, an unfortunate state of 

 affairs, but unavoidable, because some of the previous papers 

 on the subject of this study have had no bibliographies. It 

 were better, I believe, to include some duplicates than to 

 exclude some orig-inal, unduplicated records, in an effort to 

 sort out and eliminate reduplications. The present list pre- 

 sented now by the writer probably contains records of in- 

 cubation periods of more different birds than have been 

 gathered together in any single previous publication, a com- 

 parative wealth of material giving the writer an excuse for 



*Inasmuch as a great deal of the literature used in this study was 

 wholly inaccessible to the writer and had to be copied for him, he fears 

 that some errors incidental to such transcribing will have crept in, for 

 which he expresses his regret, however unavoidable on his part these 

 errors may have been. 



10 



