: Argentine Ornithology, Volume I (of 2) by P. L Sclater, W. H Hudson (books to read in your 20s female .txt) 📖

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,

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

œsophagus 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