The Cowbird and the cuckoo

For this week’s installment of Darwin’s Other Birds I give you pure Darwin, from the Voyage monograph on birds, concerning the cowbid and the cuckoo.

MOLOTHRUS NIGER
This species is common in flocks on the pasture grounds of Chile, and along the whole western shore of the southern part of the continent. In Chile it is called, according to Molina, “cureu.” It is a noisy, chattering bird, and runs in the manner of our starlings. It can be taught to speak, and is sometimes kept in cages. It builds in bushes. This Molothrus is common in large flocks on the grassy plains of La Plata, and is often mingled with the Leistes anticus, and other birds. In the same flock with the usual black kind, there were generally a few dull brown coloured ones, (Icterus sericeus of Licht.) which I presume are the young. Azara states that the brown-coloured birds are smaller than the black glossy ones, and that they sometimes form one-tenth of the whole number in a flock. In the single specimen which I brought home, the size, with the exception of the length of the wing, is only a very little less. Sonnini, in his notes to Azara, considers the brown birds as the females; I can, however, scarcely believe that so obvious a solution of the difficulty could have escaped so accurate an observer as Azara, These birds in La Plata often may be seen standing on the back of a cow or horse. While perched on a hedge, and pluming themselves in the sun, they sometimes attempt to sing or rather to hiss: the noise is very peculiar; it resembles that of bubbles of air passing rapidly from a small orifice under water, so as to produce an acute sound. Azara states that this bird, like the cuckoo, deposits its eggs in other birds’ nests. I was several times told by the country people, that there was some bird which had this habit; and my assistant in collecting, who is a very accurate person, found in the nest of the Zonotrichia ruficollis (a bird which occupies in the ornithology of S. America the place of the common sparrow of Europe), one egg larger than the others, and of a different colour and shape. This egg is rather less than that of the missel-thrush, being .93 of an inch in length, and .78 in breadth; it is of a bulky form, thick in the middle. The ground colour is a pale pinkish-white, with irregular spots and blotches of a bright reddish-brown, and others less distinct of a greyish hue. This species is evidently a very close analogue of the M. pecoris of North America, from which, however it may at once be distinguished by the absence of the glossy brown on the head, neck, and upper breast,—by the metallic blueness of its plumage in the place of a green tinge, and by its somewhat greater size in all its proportions. The young or brown-coloured specimens of these Molothri resemble each other more closely; that of the M. pecoris is of a lighter brown, especially under the throat, and the small feathers on its breast and abdomen have each an obscure dark central streak. The eggs of the Molothri, although having the same general character, differ considerably; that of the M. pecoris being smaller and less swollen in the middle; it is .85 of an inch in length, and .78 in breadth. Its colour cannot be better described than in the words of Dr. Richardson*—it is “of a greenish white, with rather small crowded and confluent irregular spots of pale liver-brown, intermixed with others of subdued purplish grey.” From this description it is obvious that the egg of M. niger is larger and of a much redder tint; the more prominent spots also are larger, the subdued grey being quite similar in both.

If we were to judge from habits alone, the specific difference between these two species of Molothrus might well be doubted; they seem closely to resemble each other in general habits,—in manner of feeding,—in associating in the same flock with other birds, and even in such peculiarities as often alighting on the backs of cattle. The M. pecoris, like the M. niger, utters strange noises, which Wilson* describes “as a low spluttering note as if proceeding from the belly.” It appears to me very interesting thus to find so close an agreement in structure, and in habits, between allied species coming from opposite parts of a great continent. Mr. Swainson† has remarked that with the exception of the Molothrus, the cuckoos are the only birds which can be called truly parasitical; namely, such as “fasten themselves, as it were, on another living animal, whose animal heat brings their young into life, whose food they alone live upon, and whose death would cause theirs during the period of infancy.” It is very remarkable, that the cuckoos and the molothri, although opposed to each other in almost every habit, should agree in this strange one of their parasitical propagation: the habit moreover is not universal in the species of either tribe. The Molothrus, like our starling, is eminently sociable, and lives on the open plains without art or disguise:‡ the cuckoo, as every one knows, is a singularly shy bird; it frequents the most retired thickets, and feeds on fruit and caterpillars. (It appears that the eggs in the same nest with that of the Molothrus pecoris, are turned out by the parent birds before they are hatched, owing to the egg of the M. pecoris being hatched in an unusually short time; in the case of the young cuckoo, as is well known, the young bird itself throws out its foster-brothers. Mr. C. Fox, however, (Silliman’s American Journal, vol. xxix. p. 292), relates an instance of three young sparrows having been found alive with a Molothrus.)

Darwin, C. R. ed. 1841. Birds Part 3 of The zoology of the voyage of H.M.S. Beagle. by John Gould. Edited and superintended by Charles Darwin. London: Smith Elder and Co.

h6 style=”text-align: right;”>The drawing is of a cowbird egg and comes from St. Nicholas Magazine for Boys and Girls, Vol. 5, July 1878, No. 9