BIRDS' NESTS 7 



for concealment (see groups of Song Sparrows, Worm-eating 

 Warbler, Black and White Warbler, Hummingbird and others) ; 

 second, by habits, whether arboreal, terrestrial or aquatic (com- 

 pare the nests of the Red-shouldered Hawk, Kingbird, Grouse, 

 Bob-white, Coot and Grebe) ; third, by haunt, whether in wood- 

 land or field, marsh or seashore, etc. (compare the nesting sites 

 of the Catbird, Field Sparrow, Clapper-Rail and Common Tern) ; 

 fourth, by temperament, whether social or solitary (compare the 

 nesting habits of the Duck Hawk and Red-shouldered Hawk with 

 those of the Cliff or Bank Swallow as shown in the groups in Halls 

 No. 204 and 208). 



TJie Nest Itself. — Probably less than twenty, possibly not 

 more than ten, per cent of the eggs laid by our smaller, perching 

 birds bring forth chicks which reach maturity. So great, there- 

 fore, is the mortality among birds in the nest that it is of vital 

 importance for a bird to select a site and build a home in which 

 its eggs may be incubated and young reared with the least pos- 

 sible exposure to the dangers which beset birds at this season. 



The character of a bird's nest depends fundamentally on the 

 following factors : 



First, Condition of Young at Birth. — Some birds are bom 

 covered w4th feathers and can swim or run about a few hours 

 after leaving the shell. These are termed prcecocial birds. 

 Others are born practically naked and pass several weeks in the 

 nest. These are termed altricial birds. With praecocial birds 

 the nest is merely an incubator in which, with the aid of heat 

 supplied by the parent bird, the eggs are hatched. With altricial 

 birds the nest is not only an incubator but a cradle as well. It is 

 obvious, therefore, that the nests of the latter class should be far 

 more complete structures than those which are to shelter young 

 birds only while their natal costume is drying. It is also to be 

 remembered that praecocial birds nest on the ground, an addi- 

 tional reason why a well-made home is not needed. 



Compare the nests and young of the Yellow-headed Black- 

 bird and Warbling Vireo with those of the common Tern, Skim- 

 mer or Black Rail and one realizes the close relation existing 

 between birds' nests and the condition in which the young leave 

 the eggs. 



