(ARGENTINE COW-BIRD.)
+Molothrus bonariensis+, _Scl. et Salv. Nomencl._ p. 37; _Hudson,
P. Z. S._ 1872, p. 809, 1874, p. 153 (Buenos Ayres); _Durnford,
Ibis_, 1877, pp. 33, 174 (Chupat); _White, P. Z. S._ 1882, p.
601 (Buenos Ayres); _Döring, Exp. al Rio Negro, Zool._ p. 41
(Carhué); _Barrows, Bull. Nutt. Orn. Cl._ viii. p. 133
(Entrerios); _Scl. Cat. B._ xi. p. 335. +Molothrus sericeus+,
_Burm. La-Plata Reise_, ii. p. 494.
_Description._--Uniform shining purplish black; less lustrous on
wings and tail; bill and feet black: total length 7·5 inches, wing
4·5, tail 3·0. _Female_ dark ashy brown, beneath paler; slightly
smaller in size.
_Hab._ Argentina, Paraguay, Bolivia, and Brazil.
This species is the _Tordo Comun_ of Azara, and is usually called
"_Tordo_" or "_Pajaro Negro_" by the Spanish, and "_Blackbird_" by
the English-speaking Argentines. A more suitable name, I think, is
the Argentine Cow-bird, which has been given to it by some writers
on ornithology, Cow-bird being the name of the closely allied
North-American species, _Molothrus pecoris_.
This Cow-bird is widely distributed in South America, and is common
throughout the Argentine country, including Patagonia, as far south as
Chupat. In Buenos Ayres it is very numerous, especially in cultivated
districts where there are plantations of trees. The male is clothed in a
glossy plumage of deep violaceous purple, the wings and tail being dark
metallic green; but seen at a distance or in the shade the bird looks
black. The female is inferior in size and has a dull, mouse-coloured
plumage, and black beak and legs. The males are much more numerous than
the females. Azara says that nine birds in ten are males; but I am
not sure that the disparity is so great as that. It seems strange and
contrary to Nature's usual rule that the smaller, shyer, inconspicuous
individuals should be in such a minority; but the reason is perhaps that
the _male eggs_ of the Cow-bird are harder-shelled than the _female
eggs_, and escape destruction oftener, when the parent bird exercises
its disorderly and destructive habit of pecking holes in all the eggs it
finds in the nests into which it intrudes.
The Cow-birds are sociable to a greater degree than most species, their
companies not breaking up during the laying-season; for, as they are
parasitical, the female merely steals away to drop her egg in any nest
she can find, after which she returns to the flock. They feed on the
ground, where in their movements and in the habit the male has of
craning out its neck when disturbed, they resemble Starlings. The male
has also a curious habit of carrying his tail raised vertically while
feeding. They follow the domestic cattle about the pastures, and
frequently a dozen or more birds may be seen perched along the back of a
cow or horse. When the animal is grazing they group themselves close to
its mouth, like chickens round a hen when she scratches up the ground,
eager to snatch up the small insects exposed where the grass is cropped
close. In spring they also follow the plough to pick up worms and grubs.
The song of the male, particularly when making love, is accompanied with
gestures and actions somewhat like those of the domestic Pigeon. He
swells himself out, beating the ground with his wings, and uttering a
series of deep internal notes, followed by others loud and clear; and
occasionally, when uttering them, he suddenly takes wing and flies
directly away from the female to a distance of fifty yards, and performs
a wide circuit about her in the air, singing all the time. The homely
object of his short-lived passion always appears utterly indifferent
to this curious and pretty performance; yet she must be even more
impressionable than most female birds, since she continues scattering
about her parasitical and often wasted eggs during four months in every
year. Her language consists of a long note with a spluttering sound, to
express alarm or curiosity, and she occasionally chatters in a low tone
as if trying to sing. In the evening, when the birds congregate on the
trees to roost they often continue singing in concert until it is quite
dark; and when disturbed at night the males frequently utter their song
while taking flight, reminding one of the _Icterus pyrrhopterus_, which
has only its usual melody to express fear and other painful emotions.
On rainy days, when they are driven to the shelter of trees, they will
often sing together for hours without intermission, the blending of
innumerable voices producing a rushing sound as of a high wind. At the
end of summer they congregate in flocks of tens of thousands, so that
the ground where they are feeding seems carpeted with black, and the
trees when they alight appear to have a black foliage. At such times
one wonders that many small species on which they are parasites do not
become extinct by means of their pernicious habit. In Buenos Ayres,
where they are most numerous, they have a migration, which is only
partial, however. It is noticeable chiefly in the autumn, and varies
greatly in different years. In some seasons it is very marked, when
for many days in February and March the birds are seen travelling
northwards, flock succeeding flock all day long, passing by with a swift
low undulating flight, their wings producing a soft musical sound; and
this humming flight of the migrating Cow-birds is as familiar to every
one acquainted with nature in Buenos Ayres as the whistling of the wind
or the distant lowing of cattle.
The procreant instinct of this _Molothrus_ has always seemed so
important to me, for many reasons, that I have paid a great deal of
attention to it; and the facts, or, at all events, the most salient of
them, which I have collected during several years of observation, I
propose to append here, classified under different headings so as to
avoid confusion and to make it easy for other observers to see at a
glance just how much I have learnt.
Though I have been familiar with this species from childhood, when I
used to hunt every day for their wasted eggs on the broad, clean walks
of the plantation, and removed them in pity from the nests of little
birds where I found them, I have never ceased to wonder at their strange
instinct, which in its wasteful destructive character, so unlike the
parasitical habit in other species, seems to strike a discordant note in
the midst of the general harmony of nature.
_Mistakes and Imperfections of the Procreant Instinct of_ Molothrus
bonariensis.
1. The Cow-birds, as we have seen, frequently waste their eggs by
dropping them on the ground.
2. They also occasionally lay in old forsaken nests. This I have often
observed, and to make very sure I took several old nests and placed them
in trees and bushes, and found that eggs were laid in them.
3. They also frequently lay in nests where incubation has actually
begun. When this happens the Cow-bird's egg is lost if incubation is far
advanced; but if the eggs have been sat on three or four days only, then
it has a good chance of being hatched and the young bird reared along
with its foster-brothers.
4. One female often lays several eggs in the same nest, instead of
laying only one, as does, according to Wilson, the _Molothrus pecoris_
of North America. I conclude that this is so from the fact that in
cases where the eggs of a species vary considerably in form, size, and
markings, each individual of the species lays eggs precisely or nearly
alike. So when I find two, three, or four eggs of the Cow-bird in one
nest all alike in colour and other particulars, and yet in half a
hundred eggs from other nests cannot find one to match with them, it is
impossible not to believe that the eggs found together, and possessing a
family likeness, were laid by the same bird.
5. Several females often lay in one nest, so that the number of eggs in
it frequently makes incubation impossible. One December I collected ten
nests of the Scissor-tail (_Milvulus tyrannus_) from my trees; they
contained a total of 47 eggs, 12 of the Scissor-tails and 35 of the
Cow-birds. It is worthy of remark that the _Milvulus_ breeds in October
or early in November, rearing only one brood; so that these ten nests
found late in December were of birds that had lost their first nests.
Probably three fourths of the lost nests of _Milvulus_ are abandoned in
consequence of the confusion caused in them by the Cow-birds.
6. The Cow-birds, male and female, destroy many of the eggs in the nests
they visit, by pecking holes in the shells, breaking, devouring, and
stealing them. This is the most destructive habit of the bird, and is
probably possessed by individuals in different degrees. I have often
carefully examined all the parasitical eggs in a nest, and after three
or four days found that these eggs had disappeared, others, newly laid,
being in their places. I have seen the female Cow-bird strike her beak
into an egg and fly away with it; and I have often watched the male bird
perched close by while the female was on the nest, and when she quitted
it seen him drop down and begin pecking holes in the eggs. In some nests
found full of parasitical eggs every egg has holes pecked in the shell,
for the bird destroys indiscriminately eggs of its own and of other
species.
_Advantages possessed by_ M. bonariensis _over its dupes._
After reading the preceding notes one might ask, If there is so much
that is defective and irregular in the reproductive instinct of _M.
bonariensis_, how does the species maintain its existence, and even
increase to such an amazing extent? for it certainly is very much more
numerous, over an equal area, than other parasitical species. For its
greater abundance there may be many reasons unknown to us. The rarer
species may be less hardy, have more enemies, be exposed to more perils
in their long migrations, &c. That it is able to maintain its existence
in spite of irregularities in its instinct is no doubt due to the fact
that its eggs and young possess many advantages over the eggs and young
of the species upon which it is parasitical. Some of these advantages
are due to those very habits of the parent bird which at first sight
appear most defective; others to the character of the egg and embryo,
time of evolution, &c.
1. The egg of the Cow-bird is usually larger, and almost invariably
harder-shelled than are the eggs it is placed with; those of the
Yellow-breast (_Pseudoleistes virescens_) being the one exception I am
acquainted with. The harder shell of its own egg, considered in relation
to the destructive egg-breaking habit of the bird, gives it the best
chance of being preserved; for though the Cow-bird never distinguishes
its own eggs, of which indeed it destroys a great many, a larger
proportion escape in a nest where many eggs are indiscriminately broken.
2. The vitality or tenacity of life appears greater in the embryo
Cow-bird than in other species; this circumstance also, in relation to
the egg-breaking habit and to the habit of laying many eggs in a nest,
gives it a further advantage. I have examined nests of the Scissor-tail,
containing many eggs, after incubation had begun, and have been
surprised at finding those of the Scissor-tail addled, even when placed
most advantageously in the nest for receiving heat from the parent bird,
while those of the Cow-bird contained living embryos, even when under
all the other eggs, and, as frequently happens, glued immovably to the
nest by the matter from broken eggs spilt over them.
The following instance of extraordinary vitality in an embryo
_Molothrus_ seems to show incidentally that in some species protective
habits, which will act as a check on the parasitical instinct, may be in
the course of formation.
Though birds do not, as a rule, seem able to distinguish parasitical
eggs from their own, however different in size and colour they may be,
they often do seem to know that eggs dropped in their nest before they
themselves have began to lay ought not to be there; and the nest, even
after its completion, is not infrequently abandoned on account of these
premature eggs. Some species, however, do not forsake their nests; and
though they do not throw the parasitical eggs out, which would seem the
simplest plan, they have discovered how to get rid of them and so save
themselves the labour of making a fresh nest. Their method is to add a
new deep lining, under which the strange eggs are buried out of sight
and give no more trouble. The _Sisopygis icterophrys_--a common
Tyrant-bird in Buenos Ayres--frequently has recourse to this expedient;
and the nest it makes being rather shallow the layer of fresh material,
under which the strange eggs are buried, is built upwards above the rim
of the original nest; so that this supplementary nest is like one saucer
placed within another, and the observer is generally able to tell from
the thickness of the whole structure whether any parasitical eggs
have been entombed in it or not. Finding a very thick nest one day,
containing two half-fledged young birds besides three addled eggs, I
opened it, removing the upper portion, or additional nest, intact, and
discovered beneath it three buried _Molothrus_ eggs, their shells
encrusted with dirt and glued together with broken egg-matter spilt over
them. In trying to get them out without pulling the nest to pieces I
broke them all; two were quite rotten, but the third contained a living
embryo, ready to be hatched, and very lively and hungry when I took it
in my hand. The young Tyrant-birds were about a fortnight old, and as
they hatch out only about twenty days after the parent-bird begins
laying, this parasitical egg with a living chick in it must have been
deeply buried in the nest for five or six weeks. Probably after the
young Tyrant-birds came out of their shells and began to grow, the
little heat from their bodies penetrating to the buried egg, served to
bring the embryo in it to maturity; but when I saw it I felt (like a
person who sees a ghost) strongly inclined to doubt the evidence of my
own senses.
3. The comparatively short time the embryo takes to hatch gives it
another and a great advantage; for, whereas the eggs of other small
birds require from fourteen to sixteen days to mature, that of the
Cow-bird hatches in eleven days and a half from the moment incubation
commences; so that when the female Cow-bird makes so great a mistake
as to drop an egg with others that have already been sat on, unless
incubation be very far advanced, it still has a chance of being hatched
before or contemporaneously with the others; but even if the others
hatch first, the extreme hardiness of the embryo serves to keep it alive
with the modicum of heat it receives.
4. Whenever the _Molothrus_ is hatched together with the young of its
foster-parents, if these are smaller than the parasite, as usually is
the case, soon after exclusion from the shell they disappear, and the
young Cow-bird remains sole occupant of the nest. How it succeeds in
expelling or destroying them, if it indeed does destroy them, I have not
been able too learn.
5. To all these circumstances favourable to the _Molothrus_ may be added
another of equal or even greater importance. It is never engaged with
the dilatory and exhaustive process of rearing its own young; and
for this reason continues in better condition than other species,
and, moreover, being gregarious and practising promiscuous sexual
intercourse, must lay a much greater number of eggs than other species.
In our domestic fowls we see that hens that never become broody lay a
great deal more than others. Some of our small birds rear two, others
only one brood in the season--building, incubation, and tending the
young taking up much time, so that they are usually from two to three
months and a half employed. But the Cow-bird is like the fowl that never
incubates, and continues dropping eggs during four months and a half.
From the beginning of September until the end of January the males are
seen incessantly wooing the females, and during most of this time eggs
are found. I find that small birds will, if deprived repeatedly of their
nests, lay and even hatch four times in the season, thus laying, if the
full complement be four, sixteen eggs. No doubt the Cow-bird lays a much
larger number than that; my belief is that every female lays from
sixty to a hundred eggs every season, though I have nothing but the
extraordinary number of wasted eggs one finds to judge from.
Before dismissing the subject of the advantages the _Molothrus_
possesses over its dupes, and of the real or apparent defects of its
instinct, some attention should be given to another circumstance, viz.,
the new conditions introduced by land-cultivation and their effect on
the species. The altered conditions have, in various ways, served to
remove many extraneous checks on the parasitical instinct, and the more
the birds multiply, the more irregular and disordered does the instinct
necessarily become. In wild districts where it was formed, and where
birds building accessible nests are proportionately fewer, the instinct
seems different from what it does in cultivated districts. Parasitical
eggs are not common in the desert, and even the most exposed nests there
are probably never overburdened with them. But in cultivated places,
where their food abounds, the birds congregate in the orchards and
plantations in great numbers, and avail themselves of all the nests,
ill-concealed as they must always be in the clean, open-foliaged trees
planted by man.
_Diversity in Colour of Eggs._
There is an extraordinary diversity in the colour, form, and disposition
of markings &c. of the eggs of _M. bonariensis_; and I doubt whether any
other species exists laying eggs so varied. About half the eggs one
finds, or nearly half, are pure unspotted white, like the eggs of birds
that breed in dark holes. Others are sparsely sprinkled with such
exceedingly minute specks of pale pink or grey, as to appear quite
spotless until closely examined. After the pure white, the most common
variety is an egg with a white ground, densely and uniformly spotted
or blotched with red. Another not uncommon variety has a very pale,
flesh-coloured ground, uniformly marked with fine characters, that look
as if inscribed on the shell with a pen. A much rarer variety has a pure
white shell with a few large or variously sized chocolate spots.
Perhaps the rarest variety is an egg entirely of a fine deep red;
but between this lovely marbled egg and the white one with almost
imperceptible specks, there are varieties without number; for there is
no such thing as characteristic markings in the eggs of this species,
although, as I have said before, the eggs of the same individual show a
family resemblance.
_Habits of the young_ M. bonariensis.
Small birds of all species, when first hatched, closely resemble each
other; after they are fledged the resemblance is less, but still
comparatively great; grey, interspersed with brown, is the colour of
most of them, or at least of the upper exposed plumage. There is also a
great similarity in their cries of hunger and fear--shrill, querulous,
prolonged, and usually tremulous notes. It is not, then, to be wondered
at that the foster-parents of the young _Molothrus_ so readily respond
to its cries, understanding the various expressions denoting hunger,
fear, pain, as well as when uttered by their own offspring. But the
young _Molothrus_ never understands the language of its foster-parents
as other young birds understand the language of their real parents,
rising to receive food when summoned, and concealing themselves or
trying to escape when the warning note is given. How does the young
_Molothrus_ learn to distinguish, even by sight, its foster-parent from
any other bird approaching the nest? It generally manifests no fear even
at a large object. On thrusting my fingers into any nest, I find young
birds, if still blind or but recently hatched, will hold up and open
their mouths expecting food; but in a very few days they learn to
distinguish between their parents and other objects approaching them,
and to show alarm even when not warned of danger. Consider the different
behaviour of three species that seldom or never warn their offspring of
danger. The young of _Synallaxis spixi_, though in a deep domed nest,
will throw itself to the ground, attempting thus to make its escape.
The young of _Mimus patagonicus_ sits close and motionless, with closed
eyes, mimicking death. The young of our common _Zenaida_, even before
it is fledged, will swell itself up and strike angrily at the intruder
with beak and wings; and, by making so brave a show of its inefficient
weapons, it probably often saves itself from destruction. But any thing
approaching the young _Molothrus_ is welcomed with fluttering wings and
clamorous cries, as if all creatures were expected to minister to its
necessities.
December 24.--To-day I found a young _Molothrus_ in the nest of
_Spermophila cærulescens_; he cried for food on seeing my hand approach
the nest; I took him out and dropped him down, when, finding himself on
the ground, he immediately made off, half-flying. After a hard chase I
succeeded in recapturing him, and began to twirl him about, making him
scream, so as to inform his foster-parents of his situation, for they
were not by at the moment. I then put him back in, or rather upon, the
little cradle of a nest, and plucked half-a-dozen large measure-worms
from an adjacent twig. The worms I handed to the bird as I drew them
from the cases, and with great greediness he devoured them all,
notwithstanding the ill-treatment he had just received, and utterly
disregarding the wild excited cries of his foster-parents, just arrived
and hovering within three or four feet of the nest.
Last summer I noticed a young Cow-bird in a stubble-field, perched on
the top of a slender dry stalk; as it was clamouring at short intervals,
I waited to see what bird would come to it. It proved to be the
diminutive _Hapalocercus flaviventris_; and I was much amused to see the
little thing fly directly to its larger foster-offspring and, alighting
on its back, drop a worm into the upturned open mouth. After remaining a
moment on its singular perch, the Flycatcher flew away, but in less than
half a minute returned and perched again on the young bird's back. I
continued watching them until the _Molothrus_ flew off, but not before I
had seen him fed seven or eight times in the same manner.
In the foregoing anecdotes may be seen the peculiar habits of the young
_Molothrus_. As the nests in which it is hatched, from those of the
little _Serpophaga_ and Wren to those of _Mimus_, vary so much in size
and materials, and are placed in such different situations, the young
_Molothrus_ must have in most of them a somewhat incongruous appearance.
But in the habits of the young bird is the greatest incongruity or
inadaptation. When the nest is in a close thicket or forest, though much
too small for the bird, and although the bird itself cannot understand
its foster-parents, and welcomes all things that, whether with good or
evil design, come near it, the unfitness is not so apparent as when the
nest is in open fields and plains.
The young _Molothrus_ differs from the true offspring of its
foster-parents in its habit of quitting the nest as soon as it is
able, trying to follow the old bird, and placing itself in the most
conspicuous place it can find, such as the summit of a stalk or weed,
and there demanding food with frequent and importunate cries. Thus the
little Flycatcher had acquired the habit of perching on the back of its
charge to feed it, because parent birds invariably perch above their
young to feed them, and the young Cow-bird prevented this by always
sitting on the summit of the stalk it perched on. The habit is most
fatal on the open and closely cropped pampas inhabited by the Cachila
(_Anthus correndera_). In December, when the Cachila Pipit rears its
second brood, the _Milvago chimango_ also has young, and feeds them
almost exclusively on the young of various species of small birds. At
this season the Chimango destroys great numbers of the young of the
Cachila and of _Synallaxis hudsoni_. Yet these birds are beautifully
adapted in structure, coloration, and habits to their station. It thus
happens that in districts where the _Molothrus_ is abundant, their eggs
are found in a majority of the Cachilas' nests: and yet to find a young
Cow-bird out of the nest is a rare thing here, for as soon as the young
birds are able to quit the nest and expose themselves they are all or
nearly all carried off by the Chimangos.
_Conjectures as to the Origin of the Parasitic Instinct in_ M.
bonariensis.
Darwin's opinion that the "immediate and final cause of the Cuckoo's
instinct is that she lays her eggs not daily, but at intervals of two
or three days" ('Origin of Species'), carries no great appearance of
probability with it; for might it not just as reasonably be said that
the parasitic instinct is the immediate and final cause of her laying
her eggs at long intervals? If it is favourable to a species with the
instinct of the Cuckoo (and it probably is favourable) to lay eggs at
longer intervals than other species, then natural selection would avail
itself of every modification in the reproductive organs that tended to
produce such a result, and make the improved structure permanent. It
is said ('Origin of Species,' chapter vii.) that the American Cuckoo
lays also at long intervals, and has eggs and young at the same
time in its nest, a circumstance manifestly disadvantageous. Of the
_Coccyzus melanocoryphus_, the only one of our three _Coccyzi_ whose
nesting-habits I am acquainted with, I can say that it never begins to
incubate till the full complement of eggs are laid--that its young are
hatched simultaneously. But if it is sought to trace the origin of the
European Cuckoo's instinct in the nesting-habits of American _Coccyzi_,
it might be attributed not to the aberrant habit of perhaps a single
species, but to another and more disadvantageous habit common to
the entire genus, viz., their habit of building exceedingly frail
platform-nests from which the eggs and young very frequently fall. By
occasionally dropping an egg in the deep, secure nest of some other
bird, an advantage would be possessed by the birds hatched in them, and
in them the habit would perhaps become hereditary. Be this as it may
(and the one guess is perhaps as wide of the truth as the other), there
are many genera intermediate between _Cuculus_ and _Molothrus_ in which
no trace of a parasitic habit appears; and it seems more than probable
that the analogous instincts originated in different ways in the two
genera. As regards the origin of the instinct in _Molothrus_, it will
perhaps seem premature to found speculations on the few facts here
recorded, and before we are acquainted with the habits of other members
of the genus. That a species should totally lose so universal an
instinct as the maternal one, and yet avail itself of that affection in
other species to propagate itself, seems a great mystery. Nevertheless,
I cannot refrain from all conjecture on the subject, and will go so far
as to suggest what may have been at least one of the many concurrent
causes that have produced the parasitic instinct. The apparently
transitional nesting-habits of several species, and one remarkable habit
of _M. bonariensis_, seem to me to throw some light on a point bearing
intimately on the subject, viz., the loss of the nest-making instinct in
this species.
Habits vary greatly; were it not so, they would never seem so well
adapted to the conditions of life as we find them, since the conditions
themselves are not unchangeable. Thus it happens that, while a species
seems well adapted to its state in its habits, it frequently seems not
so well adapted in its relatively immutable structure. For example,
without going away from the pampas, we find a Tringa with the habits of
an upland Plover, a Tyrant-bird (_Pitangus bellicosus_) preying on mice
and snakes, another Tyrant-bird (_Myiotheretes rufiventris_) Plover-like
in its habits, and finally a Woodpecker (_Colaptes campestris_) that
seeks its food on the ground like a Starling; yet in none of these--and
the list might be greatly lengthened--has there been anything like a
modification of structure to keep pace with the altered manner of life.
But, however much the original or generic habits of a species may have
become altered--the habits of a species being widely different from
those of its congeners, also a want of correspondence between structure
and habits (the last being always more suited to conditions than the
first) being taken as evidence of such alteration--traces of ancient and
disused habits frequently reappear. Seemingly capricious actions too
numerous, too vague, or too insignificant to be recorded, improvised
definite actions that are not habitual, apparent imitations of the
actions of other species, a perpetual inclination to attempt something
that is never attempted, and attempts to do that which is never
done--these and other like motions are, I believe, in many cases to be
attributed to the faint promptings of obsolete instincts. To the same
cause many of the occasional aberrant habits of individuals may possibly
be due--such as of a bird that builds in trees occasionally laying
on the ground. If recurrence to an ancestral type be traceable in
structure, coloration, language, it is reasonable to expect something
analogous to occur in instincts. But even if such casual and often
aimless motions as I have mentioned should guide us unerringly to the
knowledge of the old and disused instincts of a species, this knowledge
of itself would not enable us to discover the origin of present ones.
But assuming it as a fact that the conditions of existence, and the
changes going on in them, are in every case the fundamental cause of
alterations in habits, I believe that in many cases a knowledge of the
disused instincts will assist us very materially in the inquiry. I will
illustrate my meaning with a supposititious case. Should all or many
species of _Columbidæ_ manifest an inclination for haunting rocks and
banks, and for entering or peering into holes in them, such vague and
purposeless actions, connected with the facts that all Doves build
simple platform-nests (like _Columba livia_ and others that build on a
flat surface), also lay white eggs (the rule being that eggs laid in
dark holes are white, exposed eggs coloured), also that one species, _C.
livia_, does lay in holes in rocks, would lead us to believe that the
habit of this species was once common to the genus. We should conclude
that an insufficiency of proper breeding-places, _i. e._ new external
conditions, first induced Doves to build in trees. Thus _C. livia_ also
builds in trees where there are no rocks; but, when able, returns to its
ancestral habits. In the other species we should believe the primitive
habit to be totally lost from disuse, or only to manifest itself in a
faint uncertain manner.
Now, in _Molothrus bonariensis_ we see just such a vague, purposeless
habit as the imaginary one I have described. Before and during the
breeding-season the females, sometimes accompanied by the males, are
seen continually haunting and examining the domed nests of some of the
Dendrocolaptidæ. This does not seem like a mere freak of curiosity,
but their persistence in their investigations is precisely like that
of birds that habitually make choice of such breeding-places. It is
surprising that they never do actually lay in such nests, except when
the side or dome has been accidentally broken enough to admit the light
into the interior. Whenever I set boxes up in my trees, the female
Cow-birds were the first to visit them. Sometimes one will spend half a
day loitering about and inspecting a box, repeatedly climbing round
and over it, and always ending at the entrance, into which she peers
curiously, and when about to enter starting back, as if scared at the
obscurity within. But after retiring a little space she will return
again and again, as if fascinated with the comfort and security of such
an abode. It is amusing to see how pertinaciously they hang about the
ovens of the Oven-birds, apparently determined to take possession of
them, flying back after a hundred repulses, and yet not entering them
even when they have the opportunity. Sometimes one is seen following a
Wren or a Swallow to its nest beneath the eaves, and then clinging to
the wall beneath the hole into which it disappeared. I could fill many
pages with instances of this habit of _M. bonariensis_, which, useless
though it be, is as strong an affection as the bird possesses. That it
is a recurrence to a long disused habit, I can scarcely doubt; at least,
to no other cause that I can imagine can it be attributed; and, besides,
it seems to me that if _M. bonariensis_, when once a nest-builder, had
acquired the semiparasitical habit of breeding in domed nests of other
birds, such a habit might conduce to the formation of the instinct which
it now possesses. I may mention that twice I have seen birds of this
species attempting to build nests, and that on both occasions they
failed to complete the work. So universal is the nest-making instinct,
that one might safely say the _M. bonariensis_ had once possessed it,
and that in the cases I have mentioned it was a recurrence, too weak to
be efficient, to the ancestral habit. Another interesting circumstance
may be adduced as strong presumptive evidence that _M. bonariensis_ once
made itself an open exposed nest as _M. badius_ occasionally does--viz.,
the difference in colour of the male and female; for whilst the former
is rich purple, the latter possesses an adaptive resemblance in colour
to nests and to the shaded interior twigs and branches on which nests
are usually built. How could such an instinct have been lost? To say
that the Cow-bird occasionally dropped an egg in another bird's nest,
and that the young hatched from these accidental eggs possessed some
(hypothetical) advantage over those hatched in the usual way, and
that the parasitical habit so became hereditary, supplanting the
original one, is an assertion without any thing to support it, and
seems to exclude the agency of external conditions. Again the want
of correspondence in the habits of the young parasite and its
foster-parents would in reality be a disadvantage to the former; the
unfitness would be as great in the eggs and other circumstances; for
all the advantages the parasite actually possesses in the comparative
hardness of the egg-shell, rapid evolution of the young, &c., already
mentioned, must have been acquired little by little through the slowly
accumulating process of natural selection, but subsequently to the
formation of the original parasitical inclination and habit. I am
inclined to believe that _M. bonariensis_ lost the nest-making
instinct by acquiring that semiparasitical habit, common to so many
South-American birds, of breeding in the large covered nests of the
Dendrocolaptidæ. We have evidence that this semiparasitical habit does
tend to eradicate the nest-making one. The _Synallaxes_ build great
elaborate domed nests, yet we have one species (_S. ægithaloides_) that
never builds for itself, but breeds in the nests of other birds of the
same genus. In some species the nesting-habit is in a transitional
state. _Machetornis rixosa_ sometimes makes an elaborate nest in the
angle formed by twigs and the bough of a tree, but prefers, and almost
invariably makes choice of, the covered nest of some other species or of
a hole in the tree. It is precisely the same with our Wren, _Troglodytes
furvus_. The Yellow House-Sparrow (_Sycalis pelzelni_) invariably breeds
in a dark hole or covered nest. The fact that these three species
lay coloured eggs, and the first and last very darkly coloured eggs,
inclines one to believe that they once invariably built exposed nests,
as _M. rixosa_ still occasionally does. It may be added that those
species that lay coloured eggs in dark places construct and line their
nests far more neatly than do the species that breed in such places but
lay white eggs. As with _M. rixosa_ and the Wren, so it is with the
Bay-winged _Molothrus_; it lays mottled eggs, and occasionally builds a
neat exposed nest; yet so great is the partiality it has acquired for
large domed nests, that whenever it can possess itself of one by dint
of fighting, it will not build one for itself. Let us suppose that the
Cow-bird also once acquired the habit of breeding in domed nests, and
that through this habit its original nest-making instinct was completely
eradicated, it is not difficult to imagine how in its turn this instinct
was also lost. A diminution in the number of birds that built domed
nests, or an increase in the number of species and individuals that
breed in such nests, would involve _M. bonariensis_ in a struggle for
nests, in which it would probably be defeated. In Buenos Ayres the
White-rumped Swallow, the Wren, and the Yellow Seed-finch prefer the
ovens of the _Furnarius_ to any other breeding-place, but to obtain
them are obliged to struggle with _Progne tapera_; for this species has
acquired the habit of breeding exclusively in the ovens. They cannot,
however, compete with the _Progne_; and thus the increase of one species
has, to a great extent, deprived three other species of their favourite
building-place. Again, _Machetornis rixosa_ prefers the great nest of
the _Anumbius_; and when other species compete with it for the nest they
are invariably defeated. I have seen a pair of _Machetornis_ after they
had seized a nest attacked in their turn by a flock of six or eight
Bay-wings; but, in spite of the superior numbers, the fury of the
_Machetornis_ compelled them to raise the siege.
Thus some events in the history of our common _Molothrus_ have perhaps
been accounted for, if not the most essential one--the loss of the
nest-making instinct from the acquisition of the habit of breeding in
the covered nests of other birds, a habit that has left a strong trace
in the manners of the species, and perhaps in the pure white unmarked
eggs of so many individuals; finally, we have seen how this habit may
also have been lost. But the parasitical habit of the _M. bonariensis_
may have originated when the bird was still a nest-builder. The origin
of the instinct may have been in the occasional habit, common to so
many species, of two or more females laying together; the progenitors of
all the species of _Molothrus_ may have been early infected with this
habit, and inherited with it a facility for acquiring their present one.
_M. pecoris_ and _M. bonariensis_, though their instincts differ, are
both parasitic on a great number of species; _M. rufoaxillaris_ on _M.
badius_; and in this last species two or more females frequently lay
together. If we suppose that the _M. bonariensis_, when it was a
nest-builder, or reared its own young in the nests it seized, possessed
this habit of two or more females frequently laying together, the young
of those birds that oftenest abandoned their eggs to the care of another
would probably inherit a weakened maternal instinct. The continual
intercrossing of individuals with weaker and stronger instincts would
prevent the formation of two races differing in habit; but the whole
race would degenerate, and would only be saved from filial extinction by
some individuals occasionally dropping their eggs in the nests of other
species, perhaps of a _Molothrus_, as _M. rufoaxillaris_ still does,
rather than of birds of other genera. Certainly in this way the
parasitic instinct may have originated in _M. bonariensis_ without that
species ever having acquired the habit of breeding in the covered dark
nests of other birds. I have supposed that they once possessed it only
to account for the strange attraction such nests have for them, which
seems like a recurrence to an ancestral habit.