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May, 1917 SOME PACTORS IN THE NESTING HABIT OF BIRDS 91 pure arterial blood, by which change was brought about the wonderful develop- ment of the brain in the warm-blooded vertebrates. With development of the brain could come the cleverness necessary to the building of the average bird nest. Fig. 37a shows the circulatory system in the anterior end of a reptile (crocodile). wi?h the mixed venous and arterial blood bathing the brain, while in fig. 37b is shown the circulatory system in the anterior end of the bird, where only fresh arterial blood reaches the brain. Fig. 38 shows the difference in the development 6f the brain in the alligator (a) and in the bird (b). The main differences are the greater development of the cerebral hemispheres and the cerebellum in the bird. In the cerebral hemispheres are located all those centers which give to birds that greater intelligence which separates them at once from existing rep- tiles, while in the cerebellum are probably located those stores of great energy and centers of cobrdination that give the birds their intense activity and great cleverness. 6. As the anterior limbs of birds are highly specialized for flight, they must use for their actual nest building tools the mouth or the hind limbs. Many birds use both, though the bill is used most often, for the hind limbs are usually too highly specialized in their own way to admit of much use in nest building. In the use of the bill as a building tool lies the birds' one great handicap in ever developing nest building beyond what we see today. The great awkwardness that co'Des from having the eyes, which must gauge and judge the work, on the very base of the tool itself, will make impossible a much higher devel- opment of this art among birds. The most effi- cient tools are those such as the human hand, where the judging eye is undisturbed by the mo- tions of the tool itself, and where the eye can, as it were, remain aloof and attend exclusively to the business of overseeing. In many groups of th:? animal kingdom correlations between the tools used and the work performed are very close, but in birds, adaptation of the Fig. 38. a. BRAIN OF TtIE ALLI- GATOR SHOWING THE SMALL CEREBRAL HEMISPHERES AND SMALL CEREBELLU?I. b. BRAIN OF THE GOLDEN EAGLE? SHOWING THE LARGE CERE- BRAL ]?IEMISPHERES AND CEREBELLU?I. bill to the work of nest building is only a secondary use for this organ, the main duty of which is that of food getting. Fig. 39 shows two bills, of the Bank Swal- low (a) and of the Belted Kingfisher (b), which, while almost as dissimilar as any two bird bills can be, yet are used in constructing similar burrows in earth Fig. 39. a. HEAD OI ? A BANK SWALLOW. HEAD OF A KLXGFISIIER. banks. The key to this difference lies in the adaptation of the two bills to th,? kinds of food used. The short and frail swallow bill is used to scoop up delicate insects on the wing, while that of the kingfisher is for seizing fish in the water. 7. The remaining factors in the development of the bird into a nest building animal, are those connected with the perfection of the bill as a nest building tool. Several minor develop- ments may bc grouped under this head,