May, 1917 SOME FACTORS IN THE NESTING HABIT OF BIRDS 93 



acters, the primary food getting use of the bill overshadows all else, so that we do 

 not find that all birds with needle-like bills make compactly woven nests. This 

 form of bill merely permits such use rather than absolutely conditioning it. 



An anatomical device that perfects nest building in certain species is binocu- 

 lar vision. Not all birds are possessed of this, as the primitive condition among 

 reptiles was probably monocular vision. The common hen of the poultry yard 

 looks her friends or enemies over, first with one eye, then turning her head, checks 

 her observations with the other eye. She is handicapped by monocular vision. 

 Not so those birds that pursue active prey, for in the hawks, in the owls, in many 

 predaceous sea fowl and in most insectivorous groups the two eyes work together, 

 both focussing simultaneously on the same object, giving these fortunate ones 

 that greatest privilege of the senses, binocular vision. As a general rule vegeta- 

 ble feeders among birds do not possess this. Now of the birds pursuing active 

 prey, which possess binocular vision, those that produce the most cunningly con- 

 structed nests are found among the insectivorous forms, common examples of 

 which are the bush-tits, hummingbirds and orioles. However, binocular vision 

 does not necessarily carry with it exquisite nesting habits, because, as stated be- 

 fore, the bill is primarily adapted to food getting, so, when such vision is associ- 

 ated with a raptorial bill as in the hawks and owls, the form of the bill itself pre- 

 cludes delicate work in nest building. Other factors than the form of the bill 

 also enter into the type of nest built by birds with binocular vision. For example, 

 the Limicolae, with this type of sight, together with bills that could be used for 

 any ordinary nest building, usually construct the merest excuses of nests, fre- 

 quently not much more than a depression in the ground. In this case probably 

 the terrestrial habits of the group control the form of nest, for on the solid ground 

 there is not needed that unity of structure and thoroughness of execution essen- 

 tial to nests attached to swaying limbs many feet above the earth. Nests are 

 meager constructions or entirely lacking in the case of many predaceous sea 

 fowl, in some instances perhaps because of the hooked bill, in others because the 

 nest is secondary to the nesting site. Food being the primary consideration in a 

 bird's life, sea fowl frequently occupy cliffs and rocky islets, which, though 

 swept by every storm, are close to their fishing grounds and free from predaceous 

 mammals. Because of their exposure such places are usually barren of nesting 

 material ; in the birds inhabiting such homes we often find instead of a nest to re- 

 tain the eggs, that the latter are obovoid to the extent of being almost conical to 

 prevent them from rolling off the rocky ledges. 



Such is a brief sketch of some of the factors involved in the nesting habit of 

 birds. It is an interesting subject, and would prove a rich mine of biological ma- 

 terial to one w T ell enough acquainted with birds to make use of the great mass of 

 available data. 



Cornell University, Ithaca, New York, February 24, 1917. 



