CLASS AVES. BIRDS 



449 



Figure 318. Eggs of whip poor-will {Caprimul- 

 gus) laid on leaves in the woods. No attempt is 

 made to build a nest. (Courtesy of the American 

 Museum of Natural History.) 



European cuckoo and the American cow- 

 birds. Their eggs are usually laid in the nests 

 of birds that are smaller than themselves 

 (Fig. 319). The young birds are carefully 

 reared by their foster parents and often 

 starve or crowd out the rightful offspring. 



The eggs of birds var}' in shape, size, color, 

 and number. The smallest eggs are those of 

 certain hummingbirds, measuring less than 

 Vz inch in length; the largest eggs are those 

 of the extinct elephant bird of Madagascar, 

 Aepyornis (Fig. 333). 



As a rule, eggs laid in dark places, such 

 as those of woodpeckers, are white. Many 

 eggs are colored or spotted; those of one 

 species are in most cases distinguishable 

 from those of another species. 



Figure 319. Nest and eggs of the horned lark. 

 The nest has thick well-built walls. The coarsely 

 spotted eggs were laid in the nest by a cowbird. 

 (Photo by Hegner.) 



Figure 320. lop, yuung chmincy swifts are prac- 

 tically naked and blind at birth and are cared for 

 in the nest for about three weeks (altricial). Note 

 the nest of dried saliva and twigs. (Photo by 

 Hegner.) Bottom, piping plovers, like the newly 

 hatched ducks and pheasants, are covered with down 

 and leave the nest soon after hatching (precocial), 

 (Photo courtesy of Bertha Daubendiek.) 



The number of eggs laid in a clutch (set- 

 ting) varies in number from 1 to 20. The 

 average period of incubation for the smaller 

 passerine birds is about 12 to 14 days. The 

 eggs of the ostrich hatch in about 45 days. 

 Those of the royal albatross, a sea bird, have 

 the longest period of incubation— about 80 

 days. In some cases, only the female in- 

 cubates; in others, the male and female work 



