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.