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

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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