ecessary, as we have seen, that this should be hatched before the
other eggs; for if it were not, the old birds would stop sitting and
allow it to become cold as soon as their own young were hatched. This
danger, however, has been provided against, since the egg of the
cow-bird needs only eight or nine days of incubation, while the eggs of
those birds in whose nests it is usually found require from twelve to
fifteen days. A short time after the young cow-bird is hatched, all the
other eggs disappear, and they may sometimes be found on the ground,
broken, at a considerable distance from the nest,--so far away that the
young cow-bird could not possibly have thrown them there. The way in
which they are removed from the nest is not known, as no one has yet
watched closely enough to say whether the parents themselves destroy
them, or whether the female cow-bird returns to the nest and removes
them, to give more room for her own young when hatched.

I have already said that the smaller birds are very much disturbed and
troubled when they find one of these eggs in their nest, and are very
apt to desert it and go to another place if they have not yet any eggs
of their own. Our common yellow-bird, however, is sometimes wise enough
to find a better way out of its trouble. It values its neatly finished
nest too highly to desert it, and it is not strong enough to lift the
big egg and throw it over the edge, so it gathers a new supply of hay
and hair, and makes a false bottom to cover up the egg. Then it makes a
new lining to the nest, and lays its own eggs upon that, so that the
cow-bird's egg does not receive any of the warmth from its body, and
never hatches.

[Illustration: EGG OF COW-BIRD.]

I have given you several reasons for believing that birds are able to
think for themselves; but I do not see how anything could prove this
more clearly than this expedient of the yellow-bird for saving its young
from destruction by preventing the hatching of the cow-bird's egg.

Before leaving the subject of birds'-nests, I must say a few words about
the immense number of birds which sometimes gather in one place for the
purpose of raising their young. The enormous flocks of wild pigeons,
which from time to time visit certain parts of the United States, have a
definite portion of the woods, often several miles in extent, where they
gather every night. This is called the "roost," and here they build
their nests and rear their young. There