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). 
The 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 born 
covered with feathers and can swim or run about a few hours 
after leaving the shell. These are termed precocial birds. 
Others are born practically naked and pass several weeks in the 
nest. These are termed altricial birds. With precocial 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 precocial 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. 
