The sudden change that comes over the nature of most birds at the nesting
season is, perhaps, the most wonderful phenomenon in nature. Active,
restless birds, which normally spend the whole day on the wing, are
content to sit motionless in a cramped position upon the nest for hours
together. Birds of prey, whose nature it is to devour every helpless
creature that comes within their grasp, behave most tenderly towards
their young, actually disgorging swallowed food in order to provide them
with a meal. Timid birds become bold. Those which under ordinary
circumstances will not permit a human being to approach near them, will
sometimes, while brooding, actually allow themselves to be lifted off the
nest.
At the breeding season intelligence, which counsels self-preservation,
gives way before the parental instinct, which causes birds to expose
themselves to danger, and, in some cases, even to sacrifice their lives
for the sake of their offspring.
From the construction of the nest until the time when the young ones are
fledged the actions of the parent birds are, at any rate in the
neighbourhood of the nest, those of automata, rather than of creatures
endowed with intelligence.
On this hypothesis alone are many of the actions of nesting birds
comprehensible.
That the construction of the nest is in the main an instinctive habit and
not the result of intelligence is proved by the fact that a bird which
has been hatched out in an incubator will, at the appointed season, build
a nest. If birds were not guided by instinct they would never take the
trouble to do such a quixotic thing. What benefit can they derive from
laboriously collecting a number of twigs and weaving them into a nest?
It is, of course, natural selection that has originated this instinct;
for those species in which the parental instinct is not developed, or in
which there is not some substitute for it, must inevitably perish. When
once this instinct has taken root natural selection will tend to
perpetuate it, since those species which take the best care of their
young are those which are likely to survive in the struggle for
existence.
Many instances can be adduced to show how automatic are the actions of
birds at the nesting season.
It sometimes happens that a bird lays an egg and then proceeds to build a
nest on top of it.
Again, some birds do not know their own eggs. A whole clutch of different
ones may be substituted for those upon which the bird is sitting and the
bird will not discover the change.
The well-known bird-photographer, Mr. R. Kearton, was desirous of
obtaining a good photograph of a sitting thrush, and as he was afraid
that her eggs would be hatched before a fine, sunny day presented itself,
had some wooden dummies made. These he painted and varnished to look like
those of the thrush, and put them in the nest, wondering whether the bird
would be deceived. He need not have wondered; she would probably have sat
upon the shams even had they not been coloured.
Upon another occasion Mr. Kearton replaced some starling nestlings by his
wooden eggs, and waited to see what would happen. “In a few minutes,” he
writes, “back came the starling with a rush. She gazed in wonder at the
contents of the nest for a few seconds, but, quickly making up her mind
to accept the strangely altered condition of things, she sat down on the
bits of painted wood without a trace of discontent in either look or
action. Putting her off again, I reversed the order of things and waited.
Upon returning, the starling stared in amazement at the change that had
come over the scene during her absence; but her curiosity soon vanished,
and she commenced to brood her chicks in the most matter-of-fact way.”
Then Mr. Kearton took out the chicks and put his fist into the nest, so
that the back of his hand was uppermost. The starling actually brooded
his knuckles. We must, of course, remember that a starling’s nest is in a
hole, where there is but little light. But, provided the starling could
not see him, I believe that she would have brooded his knuckles in broad
daylight.
Crows, the most intelligent of birds, will sit upon and try to hatch golf
balls and ping-pong balls. One famous kite in Calcutta sat long and
patiently in a vain attempt to make a pill-box yield a chick, while
another member of this species subjected a hare’s skull to similar
treatment. Upon one occasion I took a robin’s egg that was quite cold and
placed it among the warm ones in a blackbird’s nest. The hen came and
brooded the egg along with her own without appearing to notice the
addition, although it was much smaller than her eggs and of a totally
different colour.
In the same way, if a set of nestlings of another species be substituted
for those already in the nest, the parent birds will usually feed the new
family without noticing the change. Instinct teaches a bird to brood all
inanimate objects it sees in the nest and to feed all living things,
whether they be its own offspring or not, and many birds blindly obey
this instinct. It is, of course, to the advantage of the species that
this should be so. For it is only on very rare occasions that foreign
objects get into a nest, and nature cannot provide for such remote
contingencies.
Similarly, instinct will not allow a bird to pay any attention to objects
outside the nest, even though these objects be the bird’s own offspring.
As everybody knows, the common cuckoo nestling ejects its foster-brethren
from the nest, and if the true parents were able to appreciate what had
happened, how much sorrow among its victims would the cuckoo cause! As a
matter of fact, no sorrow at all is caused. Incredible as it may seem,
the parent birds do not miss the young ones, nor do they appear to see
them as they lie outside the nest. In this connection I cannot do better
than quote Mr. W. H. Hudson, who was able to closely observe what
happened when a young cuckoo had turned a baby robin out of the nest.
“Here,” writes Hudson, “the young robin when ejected fell a distance of
but five or six inches, and rested on a broad, light green leaf, where it
was an exceedingly conspicuous object; and when the mother robin was on
the nest—and at that stage she was on it the greater part of the
time—warming that black-skinned, toad-like, spurious babe of hers, her
bright, intelligent eyes were looking full at the other one, just beneath
her, which she had grown in her body and had hatched with her warmth, and
was her very own. I watched her for hours; watched her when warming the
cuckoo, when she left the nest, and when she returned with food and
warmed it again, and never once did she pay the least attention to the
outcast lying there close to her. There on its green leaf it remained,
growing colder by degrees, hour by hour, motionless, except when it
lifted its head as if to receive food, then dropped it again, and when at
intervals it twitched its body as if trying to move. During the evening
even these slight motions ceased, though the feeblest flame of life was
not yet extinct; but in the morning it was dead and cold and stiff; and
just above it, her bright eyes upon it, the mother robin sat on the nest
as before warming the cuckoo.”
Even those actions of nesting birds which appear to be most intelligent
can be shown to be merely automatic. Take, for example, the curious habit
of feigning injury, which some birds have, when an enemy approaches the
young, in order to distract attention from them to itself and thus enable
them to seek cover unobserved. This surely seems a highly intelligent
act. But birds sometimes act thus before the eggs are hatched, and by so
doing actually attract attention to the eggs. This action is purely
instinctive, and is perpetuated and strengthened by natural selection
because it is beneficial to the race.
We have seen how at the nesting season all a bird’s normal actions and
instincts are subordinated to those of incubation. It is therefore but
reasonable to suppose the incubating bird to be in a very peculiar and
excitable state, a state bordering on insanity.
A bird in this condition might be expected to go into something
resembling convulsions on the approach of an enemy, and, provided its
acts under such circumstances tended to help the offspring to escape, and
were at the same time not sufficiently acute to cause the mother bird to
fall a victim to the enemy, natural selection would tend to perpetuate
and fix such actions.
Want of space prevents further dilation upon this fascinating subject.
To sum up the conclusions I desire to emphasise. A bird has during the
greater part of its life only to look after itself, and the more
intelligent it be the better will it do this, hence natural selection
tends to increase the intelligence of birds. But, at certain seasons, it
becomes all-important to the species that the adults should attend to
their young, even at risk to themselves. To secure this Nature has placed
inside birds a force, dormant at most times, which at periodic intervals
completely overrides all normal instincts, a force which compels parent
birds to rivet their attention on the nest and its contents. Thus the
sudden conversion of birds into automata is a necessity, not a mere whim
of Dame Nature. The instinct is not of very long duration; for as soon as
the young are able to fend for themselves, the parents sometimes behave
in what seems to human beings a most unnatural way: they drive off their
offspring by force. As a matter of fact, this behaviour is quite natural;
it is dictated by Nature for the benefit of the species. Strong as the
maternal instinct is, it is liable to be overridden by stronger
instincts, such as that of migration. When the time for the migratory
journey comes round, the parent birds will desert, without apparently a
pang of remorse, or even a thought, the broods for whose welfare they
have been slaving day and night. This desertion of later broods by
migratory birds is far commoner than is generally supposed. In 1826 Mr.
Blackwell inspected the house-martins’ nests under the eaves of a barn at
Blakely after the autumnal migration of these birds. Of the twenty-two
nests under the eaves inspected on 11th November, no fewer than thirteen
were found to contain eggs and dead nestlings.