P REFAC E 



In the course of certain studies in ornithology, more 

 particularly avian physiology, undertaken by the author 

 several years ago, it early became apparent that the factors 

 fixing or determining the length of the incubation period 

 with different birds was largely, if not wholly, unknown. 



The following pages give the results of a prolonged and 

 detailed study of this phase of bird physiology. 



The author regards all zoologic classification as a means^ 

 not an end: the classification adopted in this work is merely 

 a means of facilitating the handling of a mass of data full 

 of contradictions and uncertainties, and the selection of this 

 particular bird classification was governed by a question of 

 expediency only; obviously the author in no way wishes to 

 be understood as believing this classification to be the best, 

 or the only one. The author believes, however, that this 

 classification is an up-to-date reflection of our present 

 knowledge of the relation of various birds to each other. 



It is inevitable that mistakes of various sorts will be 

 found in this book; in extenuation of such errors the author 

 trusts that his critics will recall that the labor involved in 

 the investigations reviewed in the following pages was one 

 of love, and carried on in the spare moments of a fairly busy 

 professional life. 



A brief resume of pages 43 to 76 of this book was read 

 before the Annual Meeting of the American Ornithologists' 

 Union at Philadelphia, November 15, 1916. 



It is a keen pleasure here to acknowledge, with many 

 thanks, my obligations to the following friends, who have 

 generously placed at my disposal incubation data, and cog- 

 nate information, all of which has been invaluable in the 

 preparation and prosecution of this study : C. W. Beebe, 

 B. Rhett Chamberlain, L. J. Cole, E. W. Collins, L. S. Cran- 

 dall, Louis Fuertes, J. D. Figgins, W. F. Kendrick, F. H. 

 Knowlton, D. E. Lantz, F. C. Lincoln, W. DeW. Miller, E. C. 

 Murphy, J. T. Nichols, E. J. Niedrach, Clyde Phillips, 

 W. S. Pickrell, Eobert Eidgway, A. A. Saunders, Suther- 

 land Simpson, Witmer Stone, F. M. Watson, C. A. Watts 

 and A. P. Wilbur. 



The Author. 



Denver, Colo., 

 June 15, 1917. 



