\I i y, 19 17 



SOME FACTORS IN THE NESTING HABIT OF BIRDS 



89 



We may add to the above factors some others, and discuss the series as fol- 

 lows : 



1. Perhaps the first factor leading toward the care of eggs by the oviparous 

 vertebrates is the reduction of the number of eggs in a clutch and the increase in 

 the amount of yolk and food materials. This reduction in number, from the hun- 

 dreds or thousands of eggs laid by the lower vertebrates to the few laid by the 

 average reptile or bird, makes possible better care for the few. This decrease in 

 number and increase in care already appears in the fishes. Here, however, the 

 problem is complicated by the factors of en- 

 vironment, for the highly specialized pelagic 

 mackerels strew their thousands of eggs un- ^ 

 cared for on the surface of the sea, while the 

 more primitive catfishes, using the opportuni- 

 ties afforded them by the stream beds, guard 

 jealously in a nest the few hundred eggs they 

 produce each season. xk£r~\ iroviduct 



The vertebrate egg, denuded of food and 

 protective envelopes, is a single cell, which in 

 the chicken is but one twentieth of a millimeter 

 in diameter. As such these cells occur in the 

 ovary of the fowl. In the later development 

 food in the form of yolk is added inside this 

 cell until it may become more than an inch in 

 diameter, then, during its passage down the ovi- 

 duct, there are wrapped about it the nutritive 

 envelopes of albumen and the fibrous and cal- 

 careous envelopes, which we know as the egg- 

 shell. Fig. 36a shows the later development of 

 the egg in the ovary, where yolk is being added 

 to it, and the arrangement of the oviduct in 

 which those parts other than the yolk are add- 

 ed. Fig. 366 shows a hen's egg in section in Fig 36 a 0vARY AND 0VIDUCT IN 



the chicken, showing the eggs 

 of the ovary each ix its follicu- 

 lar sac where it is acquiring 

 yolk. Below the ovary the 

 glandular oviduct, which, while 

 the egg passes through it, se- 

 cretes around the egg the white 



AND THE SHELL. ft. DIAGRAM OF A 

 HEN'S EGG IN WHICH THE "EYE" OF 

 THE YOLK IS THE INCIPIENT EM- 

 BRYO, THE ONLY LIVING PART OF 

 THE EGG. 



which the eye of the yolk is the only living part. 



2. Immediately associated with this in- 

 crease in the amount of yolk material and the 

 addition of the nutritive albumen is the devel- 

 opment, first, of the fibrous, and then of the 

 calcareous, shell. This hard shell was probably 

 necessitated by the change of the reptilian an- 

 cestors from a semi-aquatic to a purely terres- 

 trial environment, for the eggs then required 

 such an impervious shell to protect them from 

 dessication. As Jthe results of a change are sel- 

 dom simple, we find that the hard shell made possible more elaborate nests, 

 and these placed in a greater variety of situations, as the developing embryo 

 was then protected from mechanical injuries as well as from drying. 



3. One of the greatest steps in advance towards avian nidification was the 

 increase and stabilization of the body temperature. This probably occurred 

 slowly as the birds became more and more differentiated from the reptiles. In- 

 dications of this slow increase still remain, as in Apteryx and others of the lowest 



