908 ANIMAL FORMS 
The eggs of birds are relatively large, and are often 
delicately colored. In some species the blotches and streaks 
of different shades are probably protective, as in the plovers 
and sandpipers, whose eggs blend perfectly with their sur- 
roundings, but many other cases exist not subject to such 
an explanation. 
The young require a high degree of heat for their devel- 
opment, and this is usually supplied by the parent. Ina 
very general way the length of sitting, or incubation, is 
proportional to the size of the egg, being from eleven to 
fourteen days in the smaller species, to seven or eight weeks 
in the ostriches. Before hatching, a sharp spine develops 
on the beak, and with this the young bird breaks its way 
through the shell. Among the quails, pheasants, plovers, 
and many other species, the young are born with a covering 
of feathers, wide-open eyes, and the ability to follow their 
parents or to make their own way in the world. Such 
nestlings are said to be precocial, in distinction to the altrical 
young of the more highly specialized species, such as the 
sparrows, woodpeckers, doves, birds of prey, and their allies, 
which are born helpless and depend for a considerable time 
on the parents for support. 
Some of the owls, crows, woodpeckers, sparrows, quails, 
etc., remain in the same localities where they are bred. 
They are resident birds. The greater number, at the ap- 
proach of winter, migrate toward the southern warmer 
climes, some species traveling in great flocks, by day or 
night, and often at immense heights. In some cases this 
movement appears to be directly related to the food-supply ; 
but there are many apparent exceptions to such a theory, 
and it is possible that many birds migrate for other reasons. 
Certain species migrate thousands of miles, along fairly 
definite routes, the young, sometimes at least, guided by 
the parents, which in turn appear to remember certain 
landmarks observed the year before. Sea-birds, in their 
journeys northward or southward, keep alongshore, occa- 
