This Buzzard does not breed on the pampas, where I have observed it, but appears there in the spring and autumn, irregularly, when migrating, and in flocks which travel in a loitering, desultory manner. The flocks usually number from thirty or forty to a hundred birds, but sometimes many more. I have seen flocks which must have numbered from one to two thousand birds. When flying the flock is very much scattered, and does not advance in a straight line, but the birds move in wide circles at a great height in the air, so that a person on horseback travelling at a canter can keep directly under them for two or three hours. On the ground one of these large flocks will sometimes occupy an area of half a square league, so widely apart do the birds keep. I have dissected a great many and found nothing but coleopterous insects in their stomachs; and indeed they would not be able to keep in such large companies when travelling if they required a nobler prey.
At the end of one summer a flock numbering about two hundred birds appeared at an estancia near my home, and though very much disturbed they remained for about three months, roosting at night on the plantation trees, and passing the day scattered about the adjacent plain, feeding on grasshoppers and beetles. This flock left when the weather turned cold; but at another estancia a flock appeared later in the season and remained all winter. The birds became so reduced in flesh that after every cold rain or severe frost numbers were found dead under the trees where they roosted; and in that way most of them perished before the return of spring.
[297.] BUTEO ERYTHRONOTUS (King).
(RED-BACKED BUZZARD.)
Buteo erythronotus, Sharpe, Cat. B. i. p. 172; Scl. et Salv. Nomencl. p. 119; Scl. P. Z. S. 1872, p. 536 (Rio Negro); Durnford, Ibis, 1877, p. 38, et 1878, p. 397 (Patagonia); Salvin, Ibis, 1880, p. 362 (Salta); Barrows, Auk, 1884, p. 109 (Azul); Withington, Ibis, 1888, p. 469 (Lomas de Zamora). Buteo tricolor, Burm. La-Plata Reise, ii. p. 436 (Mendoza and Tucuman).
Description.—Above slaty blue; wing-feathers slaty, with narrow transverse bars of black; upper tail-coverts and tail white, the latter with a broad black subapical band and numerous narrow grey cross bars: beneath white, with slight grey cross bars on the belly; bill black; feet dirty yellow: whole length 25 inches, wing 18·5, tail 10·0. Female similar, but back deep chestnut.
Hab. Southern portion of South America.
This is a fine bird—the king of South-American Buzzards. In the adult female the three colours of the plumage are strongly contrasted; the back being rusty rufous, the rest of the upper parts grey, the whole under surface pure white. It is occasionally met with in the northern provinces of the Argentine Republic, but is most common in Patagonia; and it has been said that in that region it takes the place of the nearly allied Buteo albicaudatus of Brazil. In habits, however, the two species are as different as it is possible for two raptores to be; for while the northern bird has a cowardly spirit, is, to some extent, gregarious, and feeds largely on insects, the Patagonian species has the preying habits of the Eagle, and lives exclusively, I believe, or nearly so, on cavies and other small mammals. When Captain King first discovered it in 1827, he described it as “a small beautiful Eagle.” In Patagonia it is very abundant, and usually seen perched on the summit of a bush, its broad snowy-white bosom conspicuous to the eye at a great distance—one of the most familiar features in the monotonous landscape of that grey country. The English colonists on the Chupat, Durnford says, call it the “white horse,” owing to its conspicuous white colour often deceiving them when they are out searching for strayed horses in the hills. It is a wary bird, and when approached has the habit of rising up in widening circles to a vast height in the air. When sailing about in quest of prey it usually maintains a height of fifty or sixty yards above the surface. The stomachs of all the individuals I have examined contained nothing but the remains of cavies (Cavia australis).
The nest is built on the top of a thorn bush, and is a large structure of sticks, lined with grass, fur, dry dung, and other materials. “The eggs are greyish white in colour, blotched and marked, principally towards the large end, with two shades of umber-brown” (Gould).