[Illustration]
Incubators as we know them on chicken farms are electrical gadgets with
thermostats to control the temperature, or at least with oil lamps
to supply the heat necessary for the young chick in the egg to grow.
Naturally we wouldn't expect anything so artificial as this in the bird
world, but there is one group of birds that does not brood its eggs but
has employed another method of incubating.
The birds that do this are fowl-like birds of the Australasian area.
They are variously called "mound builders" from the nest mound they
construct, "megapodes" from the large size of their scratching feet, or
bush turkeys, presumably from their edible qualities. These birds bury
their eggs and leave them thus to hatch without any brooding by the
bird. The birds have adapted their habits to two different sources of
natural heat. On some of the Pacific islands there is local volcanism
making the sand warm. To such places the birds come to bury their eggs.
But in many of the tropical forests there is not this convenient
natural heat. Another method is employed. The birds take advantage
of the heat generated by rotting vegetation. They scratch up the
surface litter of the forest floor into mounds--structures that may
be a yard or two high and five or six yards across. Some much larger
have been observed. It is into these the hens burrow and lay their
eggs. The temperatures in them have been recorded as 95° to 96° F.,
which compares with normal bird temperatures of just over 100° (bird
temperatures are a few degrees higher than normal human temperature).
The bush turkeys from Queensland have been bred in captivity, and have
given some extremely interesting data, according to an article by Mr.
Coles in the proceedings of the Zoological Society of London for 1937.
It was the male who did all the building of the mound. Though the
female started to cover the eggs laid singly in burrows in the mound,
the male finished this. And it was the male that looked after the nest
mound during the incubation period, continually scratching over the
surface layer. Both parents helped the young emerge, by digging burrows
into the mound which the emerging young, who had started to burrow out,
could use.
The young are in a very advanced state and apparently are able to fly
and look after themselves upon emerging. On the day after hatching one
chick is reported as able to flutter up to a perch six feet high.
In the captive birds mentioned above, the parents, though they were
attending to the mound and helped the chick out, appeared to take no
further interest in the chick once it was out.
There are a few other cases when birds cover or bury their eggs. With
the grebes it has been said they covered them and left them to be
incubated, but that is doubtful. Certainly the megapodes are the only
ones to present a dear case of "artificial" incubation.
This burying of eggs by the megapodes of course brings to mind the way
some reptiles, such as turtles, bury their eggs. And considering that
from an evolutionary viewpoint birds are really only modified reptiles,
it is perhaps not surprising that they too have this habit. But that
it is really an ancestral trait retained by the megapodes is doubtful.
Rather I'm inclined to think it's another example of the many ways
birds have evolved, or changed their habits so as to utilize as much of
the environment as they can in as many ways as possible.