COMPARATIVE OOLOGY OF NORTH AMERICAN BIRDS. 491 



or reached on that account. The nests are sometimes so* constructed 

 as .to hide the eggs from view; in most instances the plumage of the 

 back of the incubating- bird is in complete harmony with the environ- 

 ment of the nest, and, finally, the eggs are too small to be of any value 

 scarcely to any egg-eating animal. 



The somber-plumaged Swifts (Cvpseli), so far as is known of our 

 North American species, lay their four or live white eggs in some cav- 

 ity where they are hidden from the general view (second law, last part), 

 and in this agreeing with some of the Swallows among the Pasneres 

 that do likewise. 



There are some curious and interesting examples and departures to 

 be seen in our great and compact group of passerine birds (Passeres). 

 If we consider the Corrida' to be the most highly organized family of 

 the suborder, and Gorvus the highest genus, then in it we find the 

 species layiug somewhat numerous, dark-colored and marked eggs 

 (G.corax); but these characters rapidly change directly within the 

 family, for Gyanocephalus lays only three or four eggs, which are white, 

 tinged with greenish and profusely spotted, and such characters are con- 

 tinued into the next group, the Stumidae, where, however, they are 

 more prolific layers (S. vulgaris), and the eggs, as a rule, are not marked. 

 Then, passing by for the moment all the intermediate interrelated 

 families, we find in the Clamatores, Tyrannus, which lays but four or 

 five eggs, white, boldly and handsomely spotted with brilliant browns 

 and yet, too, in that very family (Tyrannidcv) we discover Empidonax 

 minimus laying white eggs and unspotted. But just how a Raven 

 comes to lay dark-colored, heavily marked eggs, and a small Flycatcher 

 white ones I am inclined to believe we shall never exactly know. They 

 both in this particular come under the third law, the Raven under the 

 first part of it and the Flycatcher under the second. 



The matter of coloration for protective purposes in this group would 

 hardly seem to account for the characteristic colors of the eggs of the 

 several families of passerine birds; nor, as I have heard it advanced, 

 has the light anything to do with it, although my own observations 

 lead me to think that crows are more frequently away from their nests 

 during the period of incubation than are the smaller Passercs. They are 

 not as close sitters. Such a theory, however, immediately becomes 

 untenable when wc take species like Ampelis cedrorum and Petroehelidon 

 lunifrons into consideration. The first lays a dark-colored, heavily 

 marked egg in an open nest, the parent being a close sitter, while the 

 second, a Swallow, lays a white, thickly speckled egg in. a covered 

 nest, and is not especially a close sitter. Other Swallows, which lay 

 pure white eggs in burrows (G. riparia) may fall under the operation 

 of the last part of the second law (see above). Affinity of the birds 

 again seems to have hardly anything to do with it in some cases, for 

 even among species very closely related the eggs are very different. 



