*Reprinted from Appleton's "Popular Science Monthly," with additions.
Nothing could more clearly prove that a common law runs through the
whole domain of Nature than the fact that in every division of her
realm there seems to be a class of parasites. In the vegetable world,
as is well known, there are various plants that depend wholly on other
plants for the supply of their vital forces. And in the human sphere
there are parasites in a very real and literal sense--men and women who
rely upon the toil and thrift of others to sustain them in worthless
idleness.
In view of the almost universal character of this law it would be
strange if these peculiar forms of dependence did not appear in the
avian community. We do find such developments in that department of
creation. Across the waters there is one bird that has won an
unenviable reputation as a parasite: the European cuckoo relies almost
wholly on the efforts of its more thrifty neighbors to hatch and rear
its young, and thereby perpetuate the species. Strangely enough, our
American cuckoos are not given to such slovenly habits, but build their
own nests and faithfully perform the duties of nidification, as all
respectable feathered folk should. However, this parasitical habit
breaks out, quite unexpectedly, it must be conceded, in another
American family of birds entirely distinct from the cuckoo group.
In America the cowbird, often called the cow bunting, is the only
member of the avian household that spirits its eggs into the nests of
other birds. The theory of evolution can do little toward accounting
for the anomaly, and even if it should venture upon some suggestions it
would still be just as difficult to explain the cause of the evolution
in this special group, while all other avian groups follow the law of
thrift and self-reliance.
[Illustration: Cowbird]
The cowbird belongs to the family of birds scientifically known as
_Icteridae_, which includes such familiar species as the bobolinks,
orioles, meadowlarks, and the various kinds of blackbirds, none of
which, I am glad to say, are parasites. The name _Molothrus_ has been
given to the genus that includes the cowbirds. They are confined to
the American continent, having no analogues in the lands across the
seas. The same may be said, indeed, of the whole _Icteridae_ family.
It may be a matter of surprise to many persons that there are twelve
species and subspecies of cowbirds in North and South America, for most
of us are familiar only with the common cowbird (_Molothrus ater_) of
our temperate regions. Of these twelve species only three are to be
found within the limits of the United States, one is a resident of
western Mexico and certain parts of Central America, while the rest
find habitat exclusively in South America. A fresh field of
investigation is open to some enterprising and ambitious naturalist who
wishes to study several of these species, as comparatively little is
known of their habits, and indeed much still remains to be learned of
the whole genus, familiar as one or two of the species are. Their sly,
surreptitious manners render them exceedingly difficult to study at
close range and with anything like satisfactory detail.
Are all of them parasites? Probably they are--at least to a greater or
less degree--except one, the bay-winged cowbird of South America, which
I shall reserve for notice later on in this chapter. We might assert
that our common cowbirds are the parasites _par excellence_ of the
family, for, so far as I can learn from reading and observation, they
never build their own nests or rear their own young, but shift all the
duties of maternity, save the laying of the eggs, upon the shoulders of
other innocent birds.
These avian "spongers" have a wide geographical range, inhabiting the
greater part of the United States and southern Canada, except the
extensive forest regions and some portions of the southern states.
They are most abundant in the states bordering on the upper Mississippi
River and its numerous tributaries. On the Pacific coast west of the
Cascade and Sierra Nevada mountains, they occur only as stragglers.
The most northern point at which they have been known to breed is the
neighborhood of Little Slave Lake in southern Athabaska. In the autumn
the majority of these birds migrate to southern Mexico, although a
considerable number remain in our southern states, and a few
occasionally tarry for the winter even as far north as New England and
southern Michigan.
The male cowbird looks like a well-dressed gentleman--and may have even
a slightly clerical air--in his closely fitting suit of glossy black,
with its greenish and purplish iridescence, and his hood of rich
metallic brown covering his head, neck, and chest. He makes a poor
shift as a musician, but his failure is not due to lack of effort, for
during courtship days he does his level best to sing a variety of
tunes, expanding and distorting his throat, fluffing up his feathers,
spreading out his wings and tail, his purpose evidently being to make
himself as fascinating as possible in the eyes of his lady love. One
of his calls sounds like "spreele," piped in so piercing a key that it
seems almost to perforate your brain.
One observer maintains that the cowbirds are not only parasitical in
their habits, but are also absolutely devoid of conjugal affection,
practicing polyandry, and seldom even mating. This is a serious
charge, but it is doubtless true, for even during the season of
courtship and breeding these birds live in flocks of six to twelve, the
males almost always outnumbering the females. However, if their family
relations are somewhat irregular, no one can accuse them of engaging in
brawls, as so many other birds do, for both males and females seem to
be on the most amicable terms with one another, and are, to all
appearances, entirely free from jealousy. Who has ever seen two
cowbirds fighting a duel like the orioles, meadowlarks, and robins?
In obtruding her eggs into the nests of other birds, Madame Cowbird is
sly and stealthy. She does not drive the rightful owners from their
nests, but simply watches her opportunity to drop her eggs into them
when they are unguarded. No doubt she has been on the alert while her
industrious neighbors have been constructing their domiciles, and knows
where almost every nest in the vicinity is hidden. Says Major Charles
Bendire: "In rare instances only will a fresh cowbird's egg be found
among incubated ones of the rightful owners. I have observed this only
on a single occasion." From one to seven eggs of the parasite are
found in the nests of the dupes. In most cases the number is two, but
in the case of ground builders the cowbird seems to have little fear of
overdoing her imposition. Major Bendire says that he once found the
nest of an oven-bird containing seven cowbird's eggs and only one of
the little owner's.
If parasitism were the only crime of the cowbird one would not feel so
much disposed to put her into the Newgate Calendar; but she not only
inflicts her own eggs upon her innocent victims, but often actually
tosses their eggs out of the nests in order to make room for her own.
Nor is that all; she will sometimes puncture the eggs of the owners to
prevent their hatching, and thus increase the chances of her own
offspring. Whether this is done with her beak or her claws is still an
open question, Major Bendire inclining to the belief that it is done
with the claws.
Her finesse is still further to be seen in the fact that she usually
selects some bird for a victim that is smaller than herself, so that
when her young hopefuls begin to grow they will be able to crowd or
starve out the true heirs of the family. In this way it is thought
that many a brood comes to an untimely end, the foster parents having
no means of replacing their own little ones when they have been ejected
from the nest. However, I doubt whether the cowbird's impositions are
usually so destructive as some observers are inclined to believe. I
once found a bush sparrow's nest containing one cowbird and four little
sparrows, all of which were in a thriving condition. The sparrows were
so well fed and active that as soon as I touched the nest they sprang,
with loud chirping, over the rim of their cottage and scuttled away
through the grass. They were certainly strong and healthy, in spite of
the presence of their big foster brother. Before they flitted away I
had time to notice how the little family were disposed. The cowbird
was squatted in the center of the nest, while his little foster
brothers and sisters were ranged around him, partly covering him and no
doubt keeping him snug and warm. They were further advanced than he,
for while they scrambled from the nest, he could do nothing but snuggle
close on the bottom of the cup.
A wood thrush's nest that I found contained two young thrushes and two
buntings. All of them were about half fledged. Being of nearly the
same size, the queerly assorted bantlings lived in apparent peace in
their narrow quarters. I watched them at frequent intervals, but saw
no attempts on the part of the foundlings to crowd out their
fellow-nestlings. The cowbirds were the first to leave the sylvan roof
tree. Thus it appears that the intrusion of the cowbird's eggs does
not always mean disaster to the real offspring of the brooding family,
but of course it often prevents the laying of the full complement of
eggs by the builders themselves.
Even after the youngsters have left the nest the mother cowbird does
not assume the care of them, but still leaves them in charge of the
foster parents. It is laughable, almost pathetic, to see a tiny
oven-bird or redstart feeding a strapping young cowbird which is
several times as large as herself. She looks like a pigmy feeding a
giant. In order to thrust a tidbit into his mouth she must often stand
on her tiptoes. Why the diminutive caterer does not see through the
fraud I can not say. She really seems to be attached to the hulking
youngster. By and by, however, when he grows large enough to shift for
himself, he deserts his little parents and nurses and seeks
companionship among his own blood kindred, who doubtless bring him up
in the way all cowbirds should go.
It is surprising how many species are imposed on successfully by the
cowbird. The number, so far as has been observed, is ninety, with
probably more to be added. Among the birds most frequently victimized
are the phoebes, the song sparrows, the indigo birds, the bush
sparrows, and the yellow-breasted chats. Even the nests of the
red-headed woodpecker and the rock wrens are not exempt. Some species,
notably the summer warblers, detect the imposture and set about
defeating the purposes of the interloper by building another story to
their little cottage, leaving the obtruded eggs in the cellar, where
they do not receive enough warmth to develop the embryo.
While it is surprising that acute birds should allow themselves to be
imposed on in this way, perhaps, after all, they look upon the cowbird
as a kind of blessing in disguise; at least, he may not be an unmixed
evil. They may act on the principle of reciprocity--that "one good
turn deserves another." What I mean is this: In my rambles I have
often found the cowbirds the first to give warning of the approach of a
supposed danger. Having no domestic duties of their own, they can well
secrete themselves in a tall tree overlooking the entire premises, and
thus play the useful role of sentinel. This, I am disposed to believe,
is one of the compensating uses of this parasite, and may furnish the
reason for his being tolerated in birdland. And he is tolerated. Has
any one ever seen other birds driving the cowbird away from their
breeding precincts, or charging him with desperate courage, as they do
the blue jays, the hawks, the owls, and other predatory species? He
evidently subserves some useful purpose in the avian community, or he
would not be treated with so much consideration.
A young cowbird that I purloined from the nest and tried to rear by
hand did not prove a pleasant pet. He was placed in a large cage with
several other kinds of young birds. At first he was quite docile,
taking his food from my hand and even allowing some of his feathered
companions to feed him; but in a few weeks he grew so wild and
manifested such a fierce desire for the outdoor world that I was glad
to carry him out to the woods and give him his freedom. A young
red-winged blackbird and a pair of meadowlarks developed a different
disposition.
The dwarf cow-bird (_Molothrus ater obscurus_) is similar to his
relative just described, except that he is smaller and his geographical
range is more restricted. He is a resident of Mexico, southern Texas,
southwestern Arizona, and southern California. His habits resemble
those of the common cowbird. Another bunting having almost the same
range, although a little more southerly, is the red-eyed cowbird, which
is larger and darker than our common cowbird and has the same
parasitical habits.
In South America three species have been studied by Mr. W. H. Hudson,
who, in collaboration with Mr. P. L. Sclater, has published a most
valuable work on Argentine ornithology. One of these is called the
Argentine cowbird (_Molothrus bonariensis_). It is a blue-blooded
parasite, and has been seen striking its beak into the eggs of other
birds and flying away with them. The males, it is said, show little
discrimination in pecking the eggs, for they are just as likely to
puncture the cowbird eggs as those of other birds. Every egg in a nest
is frequently perforated in this way. These buntings lay a large
number of eggs, often dropping them on the ground, laying them in
abandoned nests, or depositing them in nests in which incubation has
already begun, in which case all of them are lost. However, in spite
of this wastefulness the birds thrive, thousands of them being seen in
flocks during the season of migration.
And, by the way, a description of their habits by Mr. Hudson has thrown
interesting light on the subject of migration in the southern
hemisphere. South of the equator the recurrence of the seasons is the
exact reverse of their recurrence north of the equator, and therefore
the breeding season of the birds is in the autumn instead of the
spring; the flight from winter cold occurs in the spring instead of in
the autumn, and is toward the north instead of toward the south. Thus,
in February and March the Argentine cowbirds are seen flying in vast
battalions in the direction of the equatorial regions--that is,
northward--in whose salubrious clime they spend the winter. As our
northern autumn draws near and the southern spring approaches these
winged migrants take the air line for their breeding haunts in the
Argentine Republic and Patagonia. At the same time the migrants of the
northern hemisphere are pressing southward before the blustering north
wind. It all seems wonderful and solemn, this world-wide processional
of the seasons and the birds.
Naturally one would expect to find some other eccentricities in this
aberrant family besides that of parasitism, and in this expectation one
is not disappointed. There are two other species of cowbirds in the
Argentine country--the screaming cowbird (_Molothrus rufoaxillaris_)
and the bay-winged cowbird (_Molothrus badius_). The latter is only
partly a trencher on the rights of other birds--only half a parasite.
Indeed, it sometimes builds its own nest, which is quite a respectable
affair; but, as if to prove that it still has some remnants of cowbird
depravity in its nature, it frequently drives other birds from their
rightful possessions, appropriates the quarters thus acquired, lays its
eggs into them, and proceeds to the performance of its domestic duties
like its respectable neighbors. Its virtue is that it never imposes
the work of incubation and brood rearing on any of its feathered
associates, even though it does sometimes eject them from their
premises.
But what is to be said of the screaming cowbird? Instead of inflicting
its eggs on its more distant avian relatives, it watches its chance and
slyly drops them into the domicile of its bay-winged cousins, and
actually makes them hatch and rear its offspring! This seems to be
carrying imposture to the extreme of refinement, or possibly developing
it into a fine art, and reminds one of those human good-for-naughts who
"sponge" off their relatives rather than go among strangers.
Before closing this chapter I must call attention to one of the most
surprising discoveries ever made by an American observer of bird ways.
It was reported some time after my article on the cowbird was first
published in Appleton's "Popular Science Monthly." The observer was
Joseph F. Honecker, whose statement was printed in "American
Ornithology" for June, 1902, and runs as follows:
"As ornithologists and all bird students think and believe that the
cowbird will build no nest, but always lays in the nests of other
birds, I am glad to give the results of my experiments. In order to
get the desired results, in the spring of 1899 I secured a pair of
cowbirds and placed them in a large cage, cared well for them, and
supplied them with plenty of nesting material. To my surprise, the
female built a nest, laid four eggs, hatched them, and reared the
young, and on July twenty-eighth, young and old were given their
freedom. This will show that the cowbird will build a nest and care
for its young in captivity, while in its wild life it has never been
known to do so."