THE BIRDS 207 
pigeons and the domestic fowls, the esophagus dilates into 
a crop, in which the food is stored and softened before being 
acted upon by the gizzard. The latter is the stomach, pro- 
vided with muscular walls, especially powerful in the seed- 
eaters, and with an internal corrugated and horny lining 
which, in the absence of teeth, serves to crush the food. In 
some species, such as the domestic fowls and the pigeons, 
this process is aided by the grinding action of pebbles 
swallowed along with the food. The remaining portions, 
with pancreas and liver, vary chiefly in length, and are 
sufficiently shown in Fig. 120 to require no further descrip- 
tion. 
194. Nesting-habits.— A few birds, such as the ostriches 
and terns, merely scoop a hollow in the earth, and make no 
further pretense of constructing a nest. On the other 
hand, some birds, such as the humming-birds and pewees, 
build wonderful creations of moss, lichens, and spider-webs, 
lining it with down, and concealing it so skilfully that 
they are not often found. Every bird has its own particular 
ideas as to the fitness of its own nest, and the results are 
remarkably different, and form an interesting feature in 
studying the habits of birds. Usually the female takes 
upon herself the choice of the nest and its construction ; 
but these duties are in some species shared by the male. 
After the eggs are laid, the male may also aid in their 
incubation, or may carry food to the female. In other 
species—for example, the pigeons and many sea-birds—the 
parents take turns in sitting upon the eggs and in the sub- 
sequent care of the young. Finally, there are certain birds, 
such as the cuckoo and cowbirds, which take advantage of 
the industry of other species and deposit an egg or two in 
the nests of the latter. All the work of incubation and 
care of the young is assumed by the foster-parents, which 
sometimes neglect their own offspring in their desperate 
attempts to satisfy the appetites of the rapidly growing and 
unwelcome guests. 
