ds more attention than
this. Accordingly, the parent bird has learned to make for itself some
sort of nest, in which the young may be kept properly warm until they
are developed. The ancestral bird, who was to be the progenitor of the
entire bird class, must have had some very simple method of providing
a place in which its eggs might be hatched. As the descendants of this
original bird have passed into new situations, the various lines have
taken upon themselves different shapes until we have the multiform
birds of to-day. The habits of the birds have also varied. Each has
adapted itself to the situation in which it found itself, and no
adaptation has been more varied and effective than the adjustment of
the nesting site. Nests are found upon the ground, in the bushes, on
the lower limbs, in the crotches of the trees, in the trunks of the
trees, upon their very summits, and on the tops of inaccessible crags.
To every sort of situation some bird has been enabled to adapt itself.
This has made it possible for very many more birds to thrive than
could have found a place in the world, had they all lived upon the
same plan.

In the case of the bank swallow his nest may be a very simple
contrivance, consisting only of a tunnel running back into a bank, and
widening at the back. Some material that will soften the bed upon
which eggs are to be laid must be placed in this cavity. The whole
home is a very simple and crude affair. But little better is the
arrangement which the woodpecker calls a home. This has been cut into
the dry wood of a defective tree. No woodpecker can make his home in
absolutely solid sapwood. Hence the first labor of the woodpecker must
consist in finding a place in which it can dig. If there is an old
stump of a limb sticking up, the problem is readily solved. Such wood
has no sap in it, and is brittle enough to be easily dug out. But, if
there be no such stub, the woodpecker will find a suitable place in
most trees. At some time or other almost every tree loses a big limb.
When such accident occurs there will always be in the old trunk a
region through which sap once went to this limb. This region, deprived
of its function, goes completely dry, like the heartwood of the tree,
and it is into such material as this that the woodpecker succeeds in
drilling his well-protected home.

As birds rise higher in the scale the nest-building becomes a more
complicated affair, and after a while we find a well-woven