136 The Field Naturalist's Quarterly May 



time of hatching. Those which produce nidicolous young 

 are relatively small, whilst those of nidifugous birds are 

 large in proportion to the size of the female parent. 

 Wading-birds afford a well-known example of this rule. 

 Thirdly, birds which lay many eggs at a time must have 

 those eggs relatively small, or they would be unable to 

 successfully incubate a normal clutch. Thus we may argue 

 that "the size of an egg usually, but by no means con- 

 stantly, bears some relation to that of the parent bird." 

 The well-known habits of the cuckoo present a familiar 

 exception to this rule also : placing its eggs, as it usually 

 does, in the nests of birds much smaller than itself, it is 

 necessary that such should be the case, for if they did not 

 approximate in bulk to the eggs of the foster-parents they 

 would neither be incubated nor owned by them. 



As examples of the differently sized eggs laid by nearly 

 sized birds, the following are contrasted in the South Ken- 

 sington Museum, in cases illustrating the general characters 

 of birds' egg. Side by side are placed the eggs of the nidi- 

 fugous curlew, and of the nidicolous raven, two birds of 

 about equal size, whilst their eggs are very far out of pro- 

 portion to the size of the producers. Then come those of 

 the guillemot and the raven, where the proportion is as ten 

 to one in favour of the bird of nidifugous habits. Thus 

 reasons for these discrepancies are not far to seek, for young 

 ravens remain long in the nest, whereas young curlews and 

 guillemots are far more fully developed at the time of hatch- 

 ing, the larger egg containing a larger supply of food for the 

 greater growth of the embryo. Then, again, the number of 

 eggs laid by a raven may be six or seven, whilst the guille- 

 mot lays but one, either bird thus having about the same 

 amount of substance and surface to cover and keep warm 

 during incubation. Next, the eggs of the snipe and black- 

 bird are contrasted, the progeny of the one being able at 

 birth to run with the broken shell on its back, whilst the 

 young of the latter are born naked, blind, and helpless. 

 Then the similarity of size is shown in the size of egg laid 

 by two birds so dissimilar in bulk as the cuckoo and tree- 

 pipit, these eggs being deposited in similar nests, and the 

 young resulting from each being equally nidicolous. 



