(SCREAMING COW-BIRD.)
[Plate VI. Fig. 2.]
[Illustration: Fig. 1. MOLOTHRUS BADIUS. ad. }
" 2. " RUFO-AXILLARIS. pull. }]
+Molothrus rufoaxillaris+, _Scl. et Salv. Nomencl._ p. 37; _Hudson,
P. Z. S._ 1874, p. 161 (Buenos Ayres); _Durnford, Ibis_, 1877, p.
174 (Buenos Ayres); _White, P. Z. S._ 1882, p. 601 (Catamarca);
_Barrows, Bull. Nutt. Orn. Cl._ viii. p. 134 (Entrerios); _Scl.
Cat. B._ xi. p. 338.
_Description._--Silky black, washed with purple; wings and tail with
a slight greenish gloss; a chestnut spot on the axillaries; bill and
feet black: whole length 8·0 inches, wing 4·5, tail 3·3. _Female_
similar, but somewhat smaller.
_Hab._ Argentina and Uruguay.
This bird has no vulgar name, not being distinguished from the Common
Cow-bird by the country people. The English name of Screaming Cow-bird,
which I have bestowed on it, will, I think, commend itself as
appropriate to those who observe this bird, for they will always and
at any distance be able to distinguish it from the species it resembles
so nearly by listening to its impetuous screaming notes, so unlike
anything in the language of the Common Cow-bird.
The Screaming Cow-bird is larger than the allied species. The female is
less than the male in size, but in colour they are alike, the entire
plumage being deep blue-black, glossy, and with purple reflections; and
under the wing at the joint there is a small rufous spot. The beak is
very stout, the plumage loose, and with a strong, musky smell; the
oesophagus remarkably wide.
It is far less common than the other species of _Molothrus_, but is
not rare, and ranges south to the Buenos-Ayrean pampas, where a few
individuals are usually found in every large plantation; and, like the
_M. badius_, it remains with us the whole year. It is not strictly
gregarious, but in winter goes in parties, never exceeding five or
six individuals, and in the breeding-season in pairs. One of its most
noteworthy traits is an exaggerated hurry and bustle thrown into all its
movements. When passing from one branch to another, it goes by a series
of violent jerks, smiting its wings loudly together; and when a party of
them return from the fields they rush wildly and loudly screaming to the
trees, as if pursued by a bird of prey. They are not singing-birds; but
the male sometimes, though rarely, attempts a song, and utters, with
considerable effort, a series of chattering unmelodious notes. The
chirp with which he invites his mate to fly has the sound of a loud and
smartly-given kiss. His warning or alarm-note when approached in the
breeding-season has a soft and pleasing sound; it is, curiously enough,
his only mellow expression. But his most common and remarkable vocal
performance is a cry beginning with a hollow-sounding internal note, and
swelling into a sharp metallic ring; this is uttered with tail and wings
spread and depressed, the whole plumage raised like that of a strutting
turkey-cock, whilst the bird hops briskly up and down on its perch as if
dancing. From its puffed-out appearance, and from the peculiar character
of the sound it emits, I believe that, like the Pigeon and some other
species, it has the faculty of filling its crop with air, to use it
as a "chamber of resonance." The note I have described is quickly and
invariably followed by a scream, harsh and impetuous, uttered by the
female, though both notes always sound as if proceeding from one bird.
When on the wing the birds all scream together in concert.
The food of this species is chiefly minute seeds and tender buds; they
also swallow large caterpillars and spiders, but do not, like their
congeners, eat hard insects.
I became familiar, even as a small boy, with the habits of the Screaming
Cow-bird, and before this species was known to naturalists, but could
never find its nest, though I sought diligently for it. I could never
see the birds collecting materials for a nest, or feeding their
grown-up young like other species, and this might have made me suspect
that they did not hatch their own eggs; but it never occurred to me that
the bird was parasitical, I suppose because in summer they are always
seen in pairs, the male and female being inseparable. Probably this is
the only parasitical species in which there is conjugal fidelity. I also
noticed that, when approached in the breeding-season, the pair always
displayed great excitement and anxiety, like birds that have a nest, or
that have selected a site on which to build one. But year after year the
end of the summer would arrive, the birds reunite in parties of half a
dozen, and the mystery remain unsolved. At length, after many years,
fortune favoured me, and while observing the habits of another species
(_Molothrus badius_), I discovered by chance the procreant habits of the
Screaming Cow-birds; and as these observations throw some light on the
habits of _M. badius_, I think it best to transcribe my notes here in
full.
A pair of Leñateros (_Anumbius acuticaudatus_) have been nearly all the
winter building a nest on an acacia tree sixty yards from the house; it
is about 27 inches deep, and 16 or 18 in circumference, and appears now
nearly finished. I am sure that this nest will be attacked before long,
and I have resolved to watch it closely.
September 28.--To-day I saw a Bay-wing (_M. badius_) on the nest; it
climbed over it, deliberately inspecting every part with the critical
air of a proprietor who had ordered its construction, taking up and
rearranging some sticks and throwing others away from the nest. While
thus engaged, two common Cow-birds (_M. bonariensis_), male and female,
came to the tree; the female dropped on to the nest, and began also to
examine it, peering curiously into the entrance and quarrelling with the
first bird. After a few minutes she flew away, followed by her glossy
consort. The Bay-wing continued its strange futile work until the owners
of the nest appeared, whereupon it hopped aside in its usual slow
leisurely manner, sang for a few moments, then flew away. The similarity
in the behaviour of the two birds struck me very forcibly; in the great
interest they take in the nests of other birds, especially in
large covered nests, the two species are identical. But when the
breeding-season comes their habits begin to diverge: then the Common
Cow-bird lays in nests of other species, abandoning its eggs to their
care; while the Bay-wings usually seize on the nests of other birds
and rear their own young. Yet, as they do occasionally build a neat
elaborate nest for themselves, the habit of taking possession of the
nests of other birds is, most likely, a recently acquired one, and
probably its tendency is to eradicate the original building instinct.
October 8.--This morning, while reading under a tree, my attention was
aroused by a shrill note, as of a bird in distress, issuing from the
neighbourhood of the Leñatero's nest; after hearing it repeated at
intervals for over twenty minutes, I went to ascertain the cause. Two
Bay-wings flew up from the ground under the nest, and on searching
in the rank clover growing under the tree, I discovered the female
Leñatero, with plumage wet and draggled, trembling and appearing half
dead with the rough treatment she had experienced. I put her in the
sun, and after half an hour, hearing her mate calling, she managed to
flutter feebly away to join him. The persecutors had dragged her out
of the nest, and would, no doubt, have killed her, had I not come so
opportunely to the rescue.
Since writing the above, I have continued to watch the nest. Both the
Bay-wings and Leñateros left it for some days. Six days after picking up
the ill-treated female, the Leñateros came back and resumed possession.
Four days later the Bay-wings also came back; but on finding the
nest still occupied, they took possession of an unfinished oven of
an Oven-bird on another tree within twenty yards of the first, and
immediately began carrying in materials with which to line it. When they
had finished laying I took their five eggs, at the same time throwing
down the oven, and waited to see what their next move would be. They
remained on the spot singing incessantly, and still manifesting anxiety
when approached. I observed them four days, and then was absent from
home as many more; on returning, I found that the Leñateros had once
more disappeared, and that the nest was now held by the Bay-wings. I
also noticed that they had opened an entrance very low down at the side
of the nest which they were using; no doubt they had killed and thrown
out the young Leñateros.
It was now early in November, the height of the breeding-season, and
numbers of Common Cow-birds constantly visited the nest; but I was
particularly interested in a pair of Screaming Cow-birds that had also
began to grow fond of it, and I resolved to watch them closely. As they
spent so much of their time near the nest, showing great solicitude
when I approached it, I strongly hoped to see them breed in it, if the
Bay-wings could only be got rid of. The Screaming Cow-birds would
not, or dared not, attack them; and, as I always think that the worst
possible use one can put a little bird to is to shoot it, I could not
help them by destroying the Bay-wings. I therefore resolved to take
their eggs, hoping that that would cause them to leave in disgust.
When I was satisfied from their movements that they had finished laying,
I got up to the nest, and was astonished to find _ten_ eggs instead of
five, as I had confidently expected; for, though the Common Cow-birds
had paid a great deal of attention to the nest, I knew the Bay-wings
would not allow _them_ to lay in it.
The ten eggs in the nest were all unmistakably Bay-wing's eggs; and
having observed before that several females do occasionally lay
together, I concluded that in this case two females had laid in the
nest, though I had only seen two birds--male and female. After taking
the ten eggs the Bay-wings still remained, and in a very short time they
appeared to be laying again. When I had reason to think that the full
complement was laid, I visited the nest and found five eggs in it; these
I also took, and concluded that the second female had probably gone
away, after having been deprived of her first clutch. During all
this time the Screaming Cow-birds remained in the neighbourhood and
occasionally visited the tree; but to my very great surprise the
Bay-wings still stubbornly remained, and by-and-by I found that they
were going to lay again--the fourth time! When I next visited the nest
there were two eggs in it; I left them and returned three days later,
expecting to find five eggs, but found seven! certainly more than one
female had laid in the nest on this occasion. After taking these last
seven eggs the Bay-wings left; and though the Screaming Cow-birds
continued to make occasional visits to the nest, to my great
disappointment they did not lay in it.
April 12.--To-day I have made a discovery, and am as pleased with it as
if I had found a new planet in the sky. The mystery of the Bay-wings'
nest twice found containing over the usual complement of eggs is cleared
up, and I have now suddenly become acquainted with the procreant
instinct of the Screaming Cow-bird. I look on this as a great piece of
good fortune; for I had thought that the season for making any such
discovery was already over, as we are so near to winter.
The Bay-wings are so social in their habits that they always appear
reluctant to break up their companies in the breeding-season; no sooner
is this over, and while the young birds are still fed by the parents,
all the families about a plantation unite into one flock. About a month
ago all the birds about my home had associated in this way together, and
went in a scattered flock, frequenting one favourite feeding-spot very
much, a meadow about fifteen minutes' walk from the house. The flock was
composed, I believe, of three families, sixteen or eighteen birds in
all: the young birds are indistinguishable from the adults; but I knew
that most of these birds were young hatched late in the season, from
their incessant strident hunger-notes. I first observed them about the
middle of March. A week ago, while riding past the meadow where they
were feeding, I noticed among them three individuals with purple spots
on their plumage. They were at a distance from me, and I naturally
concluded that they were young Common Cow-birds (_M. bonariensis_),
casually associating with the Bay-wings. I was surprised to see them,
for the young male _M. bonariensis_ always acquires the purple plumage
before March, so that these individuals were changing colour five weeks
after the usual time. To-day, while out with my gun, I came upon the
flock, and noticed four of the birds assuming the purple plumage, two
of them being almost entirely that colour; but I also noticed with
astonishment that they had bay- or chestnut-coloured wings, also that
those with least purple on them were marvellously like the Bay-wings in
the mouse-coloured plumage of the body and the dark tail. I had seen
these birds before the purple plumage was acquired, and there was then
not the slightest difference amongst them, the adults and their supposed
offspring being alike; now some of them appeared to be undergoing the
process of a transmutation into another species! I at once shot the four
spotted birds along with two genuine Bay-wings, and was delighted to
find that the first were young Screaming Cow-birds.
I must now believe that the extra eggs twice found in the nest of the
Bay-wings were those of the Screaming Cow-bird, that the latter species
lays chiefly in the nests of the former, that the eggs of the two
species are identical in form, size, and colour, each bird also laying
five, and that, stranger still, the similarity is as perfect in the
young birds as it is in the eggs.
April 15.--This morning I started in quest of the Bay-wings, and
observed one individual, that had somehow escaped detection the day
before, assuming the purple dress. This bird I shot; and after the flock
had resettled a short distance off, I crept close up to them, under the
shelter of a hedge, to observe them more narrowly. One of the adults was
closely attended by three young birds; and these all, while I watched
them, fluttered their wings and clamoured for food every time the old
bird stirred on its perch. The three young birds seemed precisely alike;
but presently I noticed that one of them had a few minute purple spots,
and on shooting this one I found it to be a young _M. rufoaxillaris_,
while the other two were true young Bay-wings.
The hunger-cry of the young _M. badius_ (Bay-wing) is quite different
from that of the young _M. bonariensis_: the cry of the latter is a
long, shrill, two-syllabled note, the last syllable being prolonged into
a continuous squeal when the foster-parent approaches with food; the cry
of the young _M. badius_ is short, reedy, tremulous, and uninflected.
The resemblance of the young _M. rufoaxillaris_ to its foster-brothers
in language and plumage is the more remarkable when we reflect that the
adult bird in its habits, gestures, guttural notes, also in its deep
purple plumage, comes much nearer to _M. bonariensis_ than to _M.
badius_. It seems impossible for mimicry to go further than this. A
slight difference in size is quite imperceptible when the birds are
flying about; while in language and plumage the keenest ornithologist
would not be able to detect a difference. But it may be questioned
whether this is really a case of an external resemblance of one species
to another acquired by natural selection for its better preservation.
Possibly the young _M. rufoaxillaris_, in the first stage of its
plumage, exhibits the ancestral type--that of the progenitor of both
species. If _M. badius_ belonged to some other group--_Sturnella_ or
_Pseudoleistes_, for instance--it would scarcely be possible to
doubt that the resemblance of the young _M. rufoaxillaris_ to its
foster-brothers resulted from mimicry; but as both species belong to
the limited, well-defined group _Molothrus_, the resemblance may be
ascribed to community of descent.
Formerly I believed that though _M. badius_ is constantly seen rearing
its own young, they also occasionally dropped their eggs in the nests
of other birds. I could not doubt that this was the case after having
witnessed a couple of their young following a Yellowbreast and being
fed by it. I must now alter my opinion, for what then appeared to be
proof positive is now no proof at all, for those two birds were probably
the young of _M. rufoaxillaris_. There are, however, good reasons for
believing that _M. rufoaxillaris_ is parasitical almost exclusively
on _M. badius_. I have spoken of the many varieties of eggs _M.
bonariensis_ lays. Those of _M. badius_ are a trifle less in size,
in form elliptical, densely and uniformly marked with small spots
and blotches of dark reddish colour, varying to dusky brown; the
ground-colour is white, but sometimes, though rarely, pale blue. It is
not possible to confound the eggs of the two species. Now, ever since
I saw, many years ago, the Yellowbreast feeding the supposed young
Bay-wings, I have looked out for the eggs of the latter in other
birds' nests. I have found hundreds of nests containing eggs of _M.
bonariensis_, but never one with an egg of _M. badius_, and, I may now
add, never one with an egg of _M. rufoaxillaris_. It is wonderful that
_M. rufoaxillaris_ should lay only in the nests of _M. badius_; but
the most mysterious thing is that _M. bonariensis_, indiscriminately
parasitical on a host of species, never, to my knowledge, drops an egg
in the nest of _M. badius_, unless it be in a forsaken nest! Perhaps
it will be difficult for naturalists to believe this; for if the
_M. badius_ is so excessively vigilant and jealous of other birds
approaching its nest as to succeed in keeping out the subtle, silent,
grey-plumaged, omnipresent female _M. bonariensis_, why does it not also
keep off the far rarer, noisy, bustling, conspicuously coloured _M.
rufoaxillaris_? I cannot say. The only explanation that has occurred to
me is that _M. badius_ is sagacious enough to distinguish the eggs of
the common parasite, and throws them out of its nest. But this is
scarcely probable, for I have hunted in vain under the trees for the
ejected eggs; and I have never found the eggs of _M. badius_ with holes
pecked in the shells, which would have been the case had a _M.
bonariensis_ intruded into the nest.
With the results just recorded I felt more than satisfied, though so
much still remained to be known; and I looked forward to the next summer
to work out the rich mine on which I had stumbled by chance. Unhappily,
when spring came round again ill-health kept me a prisoner in the city,
and finding no improvement in my condition, I eventually left Buenos
Ayres at the close of the warm season to try whether change of climate
would benefit me. Before leaving, however, I spent a few days at home,
and saw enough then to satisfy me that my conclusions were correct. Most
of the birds had finished breeding, but while examining some nests of
_Anumbius_ I found one which Bay-wings had tenanted, and which for some
reason they had forsaken leaving _ten_ unincubated eggs. They were all
like Bay-wings' eggs, but I have no doubt that five of them were eggs of
_M. rufoaxillaris_. During my rides in the neighbourhood I also found
two flocks of Bay-wings, each composed of several families, and amongst
the young birds I noticed several individuals beginning to assume the
purple plumage, like those of the previous autumn. I did not think it
necessary to shoot more specimens.
The question, why _M. badius_ permits _M. rufoaxillaris_ to use its
nest, while excluding the allied parasite, _M. bonariensis_, must
be answered by future observers; but before passing from this very
interesting group (_Molothrus_) I wish to make some general remarks on
their habits and their anomalous relations to other species.
It is with a considerable degree of repugnance that we regard the
parasitical instincts in birds; the reason it excites such a feeling is
manifestly because it presents itself to the mind as--to use the words
of a naturalist of the last century, who was also a theologian, and
believed the Cuckoo had been created with such a habit--"a monstrous
outrage on the maternal affection, one of the first great dictates of
nature." An _outrage_, since each creature has been endowed with this
all-powerful affection for the preservation of its own, and not another,
species; and here we see it, by a subtle process, an unconscious
iniquity, turned from its purpose, perverted and made subservient to the
very opposing agency against which it was intended as a safeguard!
The formation of such an instinct seems indeed like an unforeseen
contingency in the system of nature, a malady strengthened, if not
induced, by the very laws established for the preservation of health,
and which the _vis medicatrix_ of nature is incapable of eliminating.
Again, the egg of a parasitical species is generally so much larger,
differing also in coloration from the eggs it is placed with, whilst
there is such an unvarying dissimilarity between the young bird and
its living or murdered foster-brothers, that, unreasoning as we know
instinct, and especially the maternal instinct, is, we are shocked at
so glaring and flagrant an instance of its blind stupidity.
In the competition for place, the struggle for its existence, said with
reason to be most deadly between such species as are most nearly allied,
the operations are imperceptible, and the changes are so gradual,
that the diminution and filial disappearance of one species is never
attributed to a corresponding increase in another more favoured species
over the same region. It is not as if the regnant species had invaded
and seized on the province of another, but appears rather as if they had
quietly entered on the possession of an inheritance that was theirs by
right. Mighty as are the results worked out by such a process, it is
only by a somewhat strained metaphor that it can be called a _struggle_.
But even when the war is open and declared, as between a raptorial
species and its victims, the former is manifestly driven by necessity.
And in this case the species preyed on are endowed with peculiar
sagacity to escape its persecutions; so that the war is not one of
extermination, but, as in a border war, the invader is satisfied with
carrying off the weak and unwary stragglers. Thus the open, declared
enmity is in reality beneficial to a species; for it is sure to cut
off all such individuals as might cause its degeneration. But we can
conceive no necessity for such a fatal instinct as that of the Cuckoo
and Cow-bird destructive to such myriads of lives in their beginning.
And inasmuch as their preservation is inimical to the species on which
they are parasitical, there must also here be a struggle. But what kind
of struggle? Not as in other species, where one perishes in the combat
that gives greater strength to the victor, but an anomalous struggle in
which one of the combatants has made his adversary turn his weapons
against himself, and so seems to have an infinite advantage. It is
impossible for him to suffer defeat; and yet, to follow out the
metaphor, he has so wormed about and interlaced himself with his
opponent that as soon as he succeeds in overcoming him he also must
inevitably perish. Such a result is perhaps impossible, as there are so
many causes operating to check the undue increase of any one species:
consequently the struggle, unequal as it appears, must continue for
ever. Thus, in whatever way we view the parasitical habit, it appears
cruel, treacherous, and vicious in the highest degree. But should we
attempt to mentally create a perfect parasitical instinct (that is, one
that would be thoroughly efficient with the least possible prejudice
to or injustice towards another species; for the preservation of the
species on which the parasite is dependent is necessary to its own) by
combining in imagination all known parasitical habits, eliminating every
offensive quality or circumstance, and attributing such others in their
place as we should think fit, our conception would probably still fall
short in simplicity, beauty, and completeness of the actual instinct of
_M. rufoaxillaris_. Instead of laying its eggs promiscuously in every
receptacle that offers, it selects the nest of a single species; so that
its selective instinct is related to the adaptive resemblance in its
eggs and young to those of the species on which it is parasitical. Such
an adaptive resemblance could not of course exist if it laid its eggs in
the nests of more than one species, and it is certainly a circumstance
eminently favourable to preservation. Then, there not being any such
incongruity and unfitness as we find in nests into which other parasites
intrude, there is no reason here to regard the foster-parents' affection
as blind and stupid; the similarity being close enough to baffle the
keenest sagacity. Nor can the instinct here appear in the light of an
outrage on the maternal affection; for the young _M. rufoaxillaris_
possesses no advantage over its foster-brothers. It is not endowed
with greater strength and voracity to monopolize the attentions of
the foster-parent or to eject the real offspring; but being in every
particular precisely like them, it has only an equal chance of being
preserved. To this wonderful parasitical instinct we may well
apply Darwin's words, when speaking of the architecture of the
hive-bee:--"Beyond this stage of perfection natural selection could
not lead."