keep them warm, while the parent birds are
away in search of food during the middle of the day. The South American
ostrich (an engraving of which is given on the next page) makes use of
the warmth of the sun and sand in the same way. According to Darwin, the
mother does not show the least affection for her young, but leaves the
labor of hatching the eggs entirely to the father, who attends to it
very faithfully, but is, of course, compelled to leave the nest
occasionally in search of food, selecting the middle of the day for this
purpose, as the heat of the sun is then sufficient to keep the eggs from
growing cold.

[Illustration: SOUTH AMERICAN OSTRICH.]

I suppose most of you know that if a quantity of wet decaying leaves or
straw is raked together into a large pile, and covered up with a thin
layer of sand or earth, and then left exposed to the sun and rain, the
heat given off by the decay of the vegetable matter forming the inside
of the pile will be retained until, after a few weeks, the interior of
the heap becomes so warm that, when the mound is broken open, a thick
cloud of smoke and steam will rise from it. The mound-building
"brush-turkey" of Australia, New Guinea, and the neighboring islands,
has somehow learned this fact; and also, that the steady and equal heat
generated is sufficient to hatch its eggs. So, instead of making a nest
and sitting upon the eggs until they are hatched, this bird, which has
very large and powerful feet, scratches up a huge pile of decaying
twigs, leaves and grass, thus making a mound often six or eight feet
high, and containing enough material to load several wagons, in which
the eggs are buried. The young birds are not helpless when hatched, like
the young of most of our singing birds, but are quite strong and active,
and able to burrow their way out of the mound, and take care of
themselves immediately.

Some birds provide for their young in still another way. They neither
sit and hatch their own eggs, nor provide an artificial incubator; but
go quietly and drop an egg into the nest of another bird, and allow this
bird to act as a nurse, hatching the egg and finding food for the young
bird. The most notable example of this habit among birds is the case of
the European cuckoo. This bird never builds a nest, or shows the least
love or even recognition of its young. The cuckoo always selects the
nest of a bird much smaller than itself, and as its eggs are much
smaller than thos