Part 11

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.

95. MOLOTHRUS RUFOAXILLARIS, Cassin.

(SCREAMING COW-BIRD.)

[Plate VI. Fig. 2.]

+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.