(SOCIABLE MARSH-HAWK.)
+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.
Concerning its breeding-habits Mr. Gibson writes:--"In the year 1873 I
was so fortunate as to find a breeding colony in one of our largest and
deepest swamps. There were probably twenty or thirty nests, placed a few
yards apart, in the deepest and most lonely part of the whole 'cañadon.'
They were slightly built platforms, supported on the rushes and two or
three feet above the water, with the cup-shaped hollow lined with pieces
of grass and water-rush. The eggs never exceeded three in a nest;
the ground-colour generally bluish white, blotched and clouded very
irregularly with dull red-brown, the rufous tint sometimes being
replaced with ash-grey."