May, 1917 



SOME FACTORS IX 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 I, 

 with the mixed venous and arterial blood bathing the brain, while in fig. 37/> 

 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 

 of the brain in the alligator (a) and in the bird (6). 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 coordination 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 

 comes 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 the 

 animal kingdom correlations between the 

 tools used and the work performed are 

 very close, but in birds, adaptation of the 

 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 (6), which, while almost as dissimilar as 

 any two bird bills can be, yet are used in constructing similar burrows in earth 



banks. The key to this difference lies 

 in the adaptation of the two bills to the 

 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 

 I,. building tool. ' Several minor develop- 

 ments may be grouped under this head, 



Fig. 38. a. Brain of the alli- 

 gator SHOWING THE SMALL 



cerebral hemispheres and 

 small cerebellum. 1). 

 Brain of the golden eagle, 

 showing the large cere- 

 bral hemispheres and 

 cerebellum. 



Fig. 39. 

 Head 



< 



a. Head of a Bank Swallow 

 of a Kingfisher. 



