272 THE BIOLOGY OF BIRDS 



When a bird lays only one egg, it is likely to be relatively 

 large, as in guillemot, gannet, and puffin, it may be noted 

 that these birds lay in places where enemies are few and 

 where it is not dangerous that the egg should have a con- 

 spicuous size. The eggs of the wingless kiwi of New Zealand 

 are larger in proportion to the bird's size than in any other 

 case ; and it is interesting to notice that they (usually two) 

 are stowed away in a nest at the end of a tunnel in the ground. 



In the case of birds whose young are hatched rapidly 

 and leave the nest very early, the eggs are usually large in 

 proportion. In other words, a relatively large legacy may 

 make for precocious development, which might be very 

 advantageous in some cases and the reverse in others. 



Professor Newton remarks that the number of eggs to 

 be covered at one time seems also to have some relation to 

 their size. Those of the snipe are equal in size to those of 

 the partridge, but the partridge is a much larger bird than 

 the snipe, therefore the snipe lays four while the partridge 

 may lay a dozen. The snipe could not cover more than four, 

 and, besides, it will cost the snipe more to produce four than 

 it will cost the partridge to produce four. Now if the number 

 of eggs bears some relation to the risks of juvenile mortality, 

 as seems generally true, and must yet be regulated in 

 reference to the number the bird can cover, we see how the 

 size of the egg might be determined, within limits, in the 

 course of natural selection. 



Observations in the poultry yard and comparisons of the 

 habits of different birds certainly suggest that a highly 

 nutritive, sluggish, anabolic bird will have larger eggs than a 

 relatively more active bird with a sparser diet. Variations 

 in constitution would be naturally correlated with diversities 

 of size and number, and these diversities would afford 

 materials for natural selection. Those constitutions in 

 which the size and the number of the eggs suited the con- 

 ditions of life would become racially fixed. 



We have noted that the eggs of ground-nesting birds are 

 in most cases relatively large, and that this may be reason- 

 ably associated with the advanced state of the young when 



