72 FALCONIM). 



An approach to the nest is always greeted by the birds with long 

 distressful cries, and this cry is also muttered in the love-season, when 

 the males often fight and pursue each other in the air. The old and 

 young birds sometimes live together until the following spring. 



307. ROSTRHAMUS SOCIABILIS (Vieill.). 

 (SOCIABLE MARSH-HAWK.) 



Rostrhamus sociabilis, Scl. et Salv. Nomencl. p. 121 ; iid. P. Z. S. 1869, p. 1GO 

 (Buenos Ay res) ; Dumford, Ibis, 1877, p. 188 (Buenos Ay res) ; Gibson, 

 Ibis, 1879, p. 413 (Buenos Ayres) ; Withinyton, Ibis, 1888, p. 470 (Lomas 

 de Zamora). Rostrhamus leucopygus, Sharpe, Cat. B. i. p. 328. Rostr- 

 hamus hamatus, Burm. La-Plata Reise, ii. p. 435 (Rio Parana). 



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, wing 13'0, tail 7*5. Female similar, but rather larger. 



Hob. 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 (Am- 

 pullaria), 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 

 vcrtebrfe of the tail must have been somewhat modilicd by such a habit. 



