CLASS AVES 625 



lined hollow in the ground. The great horned owl lays its eggs 

 in an old hawk's or squirrel's nest. The mourning-dove builds 

 a loose platform of twigs. There, are all stages of complexity 

 between this simple attempt and the beautifully woven, hanging 

 nest of the Baltimore oriole. Certain features distinguish the 

 nest of one bird from that of another; thus the nest of the chip- 

 ping sparrow almost invariably contains a lining of horsehair, 

 that of the shrike contains feathers, that of the American gold- 

 finch is lined with thistle-down, and the nests of the ruby- 

 throated humming-bird and the wood pewee are covered exter- 

 nally with lichens. 



A few birds not only do not build nests, but even refuse to 

 incubate their eggs and take care of their offspring. This is 

 true of the European cuckoo and the American cow-bird. The 

 breeding habits of the latter are very interesting. There are 

 more male cowbirds than females and each female therefore mates 

 with several males, a condition known as polyandry. The 

 females seek out the nests of other birds, usually those smaller 

 than themselves, in which to lay their eggs. The young cow- 

 birds are carefully reared by their foster parents, and often starve 

 out the rightful owners. 



The eggs of birds vary in size, color, and number. The small- 

 est eggs are those of certain humming-birds, measuring less than 

 half an inch long; the largest eggs are those of the extinct ele- 

 phant-birds of Madagascar, dLpyornis, which measure over thir- 

 teen inches in length (see p. 598). 



As a rule, eggs laid in dark places, such as those of the bank- 

 swallow, kingfisher, woodpecker, and owl, are white. Many 

 eggs are colored, some possessing a uniform ground color; others, 

 spots of various hues; and still others, both a ground color and 

 spots. These colors usually vary but slightly in the eggs laid 

 by different individuals of the same species, and those of one 

 species are, in most cases, easily distinguished from those of an- 

 other species. 



The eggs laid at a setting vary in number from one to about 



2 S 



