DiTisioa or Birls 



A Study 



OF THE 



Incubation Periods 



OF 



Birds 



WHAT DETERMINES THEIR 

 LENGTHS ? 



W. H. BERGTOLD, M. D., M. Sc 



Member of the American Ornithologists'' Union 



THE KENDRICK-BELLAMY CO. 



DENVER, COLORADO 

 1917 



