90 



THE CONDOR 



Vol. XIX 



living birds the body temperature is at an average of from 5 to 10 degrees 

 lower than in the more highly specialized forms such as our songbirds, where it 

 may reach 111 degrees F. AVith this increase in temperature the guarding of the 

 nest by the parents became more and more a definite act of incubation in which 

 the heat was furnished by the body of the parent. Thus the hatching was speeded 

 up and removed from the uncertainties of weather or the irregularities of decom- 

 posing vegetable matter. It is interesting to note at this point that the Mega- 

 podes, which leave their eggs to be incubated by the warm sand or by decaying 

 vegetable matter, of which group I have already mentioned the Australian Brush 

 Turkey, nearly all occupy islands in the Australian region where there are few 

 or no small predaceous mammals. This style of nesting would be of little protec- 

 tion against such enemies, for the mounds are conspicuous, the eggs are rela- 

 tively large, and incubation is slow. The heat is low and the young have to be 

 advanced enough at hatching to burrow out of the sand or mass of debris and 



hustle a living for themselves at once. We 

 might speculate that this simple form of 

 nesting and those birds that practiced it 

 have been eliminated in other parts of the 

 world by some such enemy. 



4. A corollary to this increase in body 

 temperature is the development of the rep- 

 tilian scales as a whole or in part into the 

 delicate fimbriated scales of birds which we 

 know r as feathers. These made a high body 

 heat possible, and made the heat for incuba- 

 tion more reliable, as they helped cover the 

 eggs. Incidentally they made flight possible 

 in this group, and with flight the high devel- 

 opment of arboreal nests so characteristic of 

 the more specialized forms. Birds were 

 probably evolved from the bipedal Dinosaurs 

 and came into existence without that ability 

 to climb which is the birthright of the small 

 quadrupeds, w 7 here the possession of both 

 fore and hind feet in a generalized form 

 mechanically fits the owner for an arboreal 

 life. Instead birds probably hopped awk- 

 wardly from limb to limb, flight at first being merely a lengthening of such 

 leaps. The scansorial birds, such as woodpeckers, creepers, nuthatches, certain 

 parrots and others, are highly specialized groups in which the climbing habit 

 has been only recently redeveloped. 



The preceding factors are especially associated with the actual incubation of 

 the eggs. The succeeding are more immediately associated witk the art of nest 

 building itself. 



5. Coming at this same time of transition, when the birds were differenti- 

 ated from their reptilian ancestors, was the change in the heart from a 'three- 

 chambered to a perfect four-chambered organ. Thus in the anterior end of the 

 body were the venous and arterial bloods entirely separated. This great advance 

 in the circulatory system meant that only the freshest and purest blood went to 

 the brain, the delicate cells of this organ being stimulated and nourished by the 



Fig. 37. a. ClKCULATORY SYSTEM! IX 

 THE ANTERIOR END OF A REPTILE, 

 WITH THE MIXED VENOUS AND ARTE- 

 RIAL BLOOD BATHING THE BRAIN. 



ft. Circulatory system in the 



ANTERIOR END OF A BIRD, IN WHICH 

 ONLY FRESH ARTERIAL BLOOD 



REACHES THE BRAIN. 



