SELBORNE, _April_ 3_rd_, 1776.
Dear Sir,--Monsieur Herissant, a French anatomist, seems persuaded that
he has discovered the reason why cuckoos do not hatch their own eggs; the
impediment, he supposes, arises from the internal structure of their
parts, which incapacitates them for incubation. According to this
gentleman, the crop, or craw, of a cuckoo does not lie before the sternum
at the bottom of the neck, as in the _gallinae_, _columbae_, etc., but
immediately behind it, on and over the bowels, so as to make a large
protuberance in the belly.
Induced by this assertion, we procured a cuckoo; and, cutting open the
breast-bone, and exposing the intestines to sight, found the crop lying
as mentioned above. This stomach was large and round, and stuffed hard,
like a pincushion, with food, which, upon nice examination, we found to
consist of various insects; such as small scarabs, spiders, and dragon-
flies; the last of which we have seen cuckoos catching on the wing as
they were just emerging out of the aurelia state. Among this farrago
also were to be seen maggots, and many seeds, which belonged either to
gooseberries, currants, cranberries, or some such fruit; so that these
birds apparently subsist on insects and fruits; nor was there the least
appearance of bones, feathers, or fur, to support the idle notion of
their being birds of prey.
The sternum in this bird seemed to us to be remarkably short, between
which and the anus lay the crop, or craw, and immediately behind that the
bowels against the back-bone.
It must be allowed, as this anatomist observes, that the crop placed just
upon the bowels must, especially when full, be in a very uneasy situation
during the business of incubation; yet the test will be to examine
whether birds that are actually known to sit for certain are not formed
in a similar manner. This inquiry I proposed to myself to make with a
fern-fowl, or goat-sucker, as soon as opportunity offered: because, if
their formation proves the same, the reason for incapacity in the cuckoo
will be allowed to have been taken up somewhat hastily.
Not long after a fern-owl was procured, which, from its habit and shape,
we suspected might resemble the cuckoo in its internal construction. Nor
were our suspicions ill-grounded; for, upon the dissection, the crop, or
craw, also lay behind the sternum, immediately on the viscera, between
them and the skin of the belly. It was bulky, and stuffed hard with
large _phalaenae_, moths of several sorts, and their eggs, which no doubt
had been forced out of those insects by the action of swallowing.
Now as it appears that this bird, which is so well known to practise
incubation, is formed in a similar manner with cuckoos, Monsieur
Herissant's conjecture, that cuckoos are incapable of incubation from the
disposition of their intestines, seems to fall to the ground; and we are
still at a loss for the cause of that strange and singular peculiarity in
the instance of the _cuculus canorus_.
We found the case to be the same with the ring-tail hawk, in respect to
formation; and, as far as I can recollect, with the swift; and probably
it is so with many more sorts of birds that are not granivorous.