CLASS AVES 403 



tions, Arctic terns cover annually a distance of more than 20,000 miles. 

 Another bird which takes a long journey is the golden plover, which 

 nests in the barren grounds of the northern part of British America 

 and winters in southern Brazil and Argentina. Its journey involves a 

 round trip of more than 16,000 miles. This trip is taken also by a few 

 other shore birds. 



How birds find their way has never been satisfactorily determined. 

 This faculty seems to rest upon a very accurate sense of location which 

 enables them to fly practically in a straight line from one point to another, 

 even though darkness or fog may hide any features that would guide 

 them. 



433. Reproduction. — Birds may mate either for the rearing of a 

 single brood or for life. The nesting locality seems to be in most cases 

 chosen by the male, though it is the female which determines the precise 

 location of the nest. One or both members of a pair take part in' its 

 construction, which varies greatly among different types of birds. Some 

 birds make no nest at all but lay their eggs directly upon the ground. 

 Others make crude nests by scratching out hollows and then utiUzing 

 a few pebbles, twigs, or bits of grass. At the other extreme in nest 

 building are the beautifully woven pendant nests of orioles, especially 

 of certain tropical ones, w^hich in many cases are several feet in length. 

 Some birds do not rear their own young but deposit their eggs in the 

 nests of others who become foster parents. This is true of the European 

 cuckoo and of some American cowbirds. In these species mating of 

 one bird with a single one of the opposite sex does not take place, for 

 each female mates with many males, a phenomenon known as polyandry. 

 On the other hand, as in the case of ostriches and of some fowls, one male 

 may mate with several females, and this is known as polygamy. 



Birds' eggs vary greatly in size, color, and number. The size of the 

 egg bears a general relationship to the size of the bird, while the number 

 is greatest in those birds whose nests are most exposed to destruction. 

 The color of eggs is related somewhat to the place where they are laid. 

 Those laid in cavities are usually white, or white with reddish spots; 

 those upon the ground are usually streaked or mottled in such a way 

 as to resemble the surroundings; and those in nests in trees and bushes are 

 frequently blue or bluish white with markings of various patterns. 



The time of incubation varies somewhat with the size of the bird, 

 the cowbird having an incubation period of only about ten days; the 

 ordinary song bird, about two weeks; fowls, three weeks; ducks, geese, 

 and swans, from three to five weeks; the ostriches, from seven to eight 

 and a half weeks; and the Austrahan emu, ten or eleven weeks. Usually 

 the duty of incubation is assumed mainly by the female, and in other 

 cases the two share in it. In a few instances, however, the male does 

 all of the incubating. The last is true in the case of the American ostrich 



