Page:Condor19(3).djvu/19

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 ?Jmse 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 v/sion. 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 arc meager constructions or entirely lacking in the case of many predaceous sca 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 ?nhabiting such homes we often find instead of a nest to re- tain tlqe 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 well enough acquainted with birds to make use of the great mass of available data. Cornell University, Ithaca, New York, February 24, 1917.