+Rostrhamus sociabilis+, _Scl. et Salv. Nomencl._ p. 121; _iid. P. Z. S._ 1869, p. 160 (Buenos Ayres); _Durnford, Ibis_, 1877, p. 188 (Buenos Ayres); _Gibson, Ibis_, 1879, p. 413 (Buenos Ayres); _Withington, Ibis_, 1888, p. 470 (Lomas de Zamora). +Rostrhamus leucopygus+, _Sharpe, Cat. B._ i. p. 328. +Rostrhamus hamatus+, _Burm. La-Plata Reise_, ii. p. 435 (Rio Paraná). _Description._--Blackish slate-colour; head and wing-feathers black; rump white; tail white, with a broad band occupying the apical half, but leaving the tail end greyish; bill orange, apical half black; feet orange-brown, claws black: whole length 17·0 inches, wings 13·0, tail 7·5. _Female_ similar, but rather larger. _Hab._ South America. This Hawk in size and manner of flight resembles a Buzzard, but in its habits and the form of its slender and very sharply hooked beak it differs widely from that bird. The name of Sociable Marsh-Hawk, which Azara gave to this species, is very appropriate, for they invariably live in flocks of from twenty to a hundred individuals, and migrate and even breed in company. In Buenos Ayres they appear in September and resort to marshes and streams abounding in large water-snails (_Ampullaria_), on which they feed exclusively. Each bird has a favourite perch or spot of ground to which it carries every snail it captures, and after skilfully extracting the animal with its curiously modified beak, it drops the shell on the mound. When disturbed or persecuted by other birds they utter a peculiar cry, resembling the shrill neighing of a horse. In disposition they are most peaceable, and where they are abundant all other birds soon discover that they are not as other Hawks are and pay no attention to them. When soaring, which is their favourite pastime, the flight is singularly slow, the bird frequently remaining motionless for long intervals in one place; but the expanded tail is all the time twisted about in the most singular manner, moved from side to side, and turned up until its edge is nearly at a right angle with the plane of the body. These tail-movements appear to enable it to remain stationary in the air without the rapid vibratory wing-motions practised by _Elanus leucurus_ and other hovering birds; and I should think that the vertebræ of the tail must have been somewhat modified by such a habit.