and have to be fed and brooded by their
mothers for a long time. Other young birds, like young owls, falcons
(fig. 1), and hawks, also leave the egg blind and helpless, but their
bodies are covered with long woolly down. Until quite recently no one
could say why these differences should be, but at last we are beginning
to see a way out of the puzzle. There seems to be no doubt that once
upon a time the young of all birds left the shell in a fully active
state, and clothed in down; further, we know that these early birds were
reared in nurseries amid the tree-tops, and climbed about the branches
by means of their legs and beaks, aided by claws in their wings, till at
last their feathers grew and replaced the down, and they were able to
fly. In course of time some birds took to building their nests on the
ground, perhaps because so many young perished every year by falling
from the trees. On the ground this danger was overcome. But, among those
which chose to stay in the trees, a change was introduced. They took to
laying smaller eggs, containing less food; in consequence, the young
were hatched before they had reached such a forward state of development
as their cousins on the ground; and though this meant far more work for
the parents, who had to feed their helpless and blind little ones, the
change proved beneficial, because, being helpless, they remained quietly
in the nest till their feathers grew, and then they were in no danger of
falling, for they saved themselves by flight. These two devices proved
so successful that they are followed still--probably always will be. The
fact that many young birds which are quite helpless are now reared in
nurseries on the ground, as in the case of young skylarks (fig. 3), is a
fact of interest; for it shows that the parents have chosen this nesting
site comparatively recently, and are of course unable to lay large eggs,
which shall produce active young, like young chickens, at will. They
have acquired the habit, so to speak, of laying small eggs, and cannot
alter it by changing their nesting-place.

[Illustration: Fig. 4.--Tern, with young one.]

Most young birds which leave the eggs in a forward condition have the
down which clothes them curiously striped. This is a device which
enables the young bird to resemble the grass and herbage with which it
is surrounded, and so escape the eye of prowling birds and beasts of
prey. The dark stripes at a little distance look like shadows