Fam. XI. ICTERIDae, or TROUPIALS.
The Icteridae or Troupials const.i.tute a large group of Pa.s.serine birds allied to our Starlings (_Sturnidae_), of which they take the place in the New World. They are at once structurally distinguishable from the Starlings by having only 9 primaries in the wing, just as the Mniotiltidae of the New World are in a similar manner distinguishable from the Sylviidae.
In America the Icteridae play an important part, numbering some 130 species, and extending throughout the two continents from north to south. Of these, 15 species occur in Argentina, and amongst them are three species of Cow-bird (_Molothrus_), remarkable for their parasitic habits, of which Hudson"s observations have enabled him to give a full and, for the first time, a tolerably complete account.
93. AMBLYCERCUS SOLITARIUS (Vieill.).
(SOLITARY Ca.s.sIQUE.)
+Ca.s.sicus solitarius+, _Scl. et Salv. Nomencl._ p. 36; _Barrows, Bull. Nutt. Orn. Cl._ viii. p. 133 (Entrerios); _Scl. Cat. B._ p. 326.
_Description._--Uniform black; bill white; feet black: total length 11 inches, wings 48, tail 45. _Female_ similar, but smaller.
_Hab._ Peru, Bolivia, Brazil, Paraguay, and Northern Argentina.
Mr. Barrows obtained a single specimen of this species at Concepcion, and others were seen. It was said to be an excellent song-bird, and to be more abundant further up the Uruguay River.
94. MOLOTHRUS BONARIENSIS (Gm.).
(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); _Doring, Exp. al Rio Negro, Zool._ p. 41 (Carhue); _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 l.u.s.trous on wings and tail; bill and feet black: total length 75 inches, wing 45, tail 30. _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-sh.e.l.led 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 s.n.a.t.c.h up the small insects exposed where the gra.s.s 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 pa.s.sion 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, pa.s.sing 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, cla.s.sified 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 sh.e.l.ls, 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 sh.e.l.l, 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-sh.e.l.led 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 sh.e.l.l 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 circ.u.mstance 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 sh.e.l.ls 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 sh.e.l.ls 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 modic.u.m 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 sh.e.l.l 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 circ.u.mstances 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 s.e.xual 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 circ.u.mstance, 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 spa.r.s.ely 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 sh.e.l.l with a pen. A much rarer variety has a pure white sh.e.l.l 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 caerulescens_; 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.