_Description._--Above grey; lesser wing-coverts and scapulars black; tail white, two central rectrices grey: beneath white; bill black; feet yellow; claws yellow: whole length 14·5 inches, wing 11·0, tail 7·0. _Female_ similar, but rather larger. _Hab._ Central and South America. This interesting Hawk is found throughout the Argentine Republic, but is nowhere numerous. It also inhabits Chili, where, Gay says, it is called _Bailarin_ (dancer) on account of its aerial performances. It is a handsome bird, with large ruby-red irides, and when seen at a distance its snow-white plumage and buoyant flight give it a striking resemblance to a gull. Its wing-power is indeed marvellous. It delights to soar, like the Martens, during a high wind, and will spend hours in this sport, rising and falling alternately, and at times, seeming to abandon itself to the fury of the gale, is blown away like thistle-down, until, suddenly recovering itself, it shoots back to its original position. Where there are tall poplar trees these birds amuse themselves by perching on the topmost slender twigs, balancing themselves with outspread wings, each bird on a separate tree, until the tree-tops are swept by the wind from under them, when they often remain poised almost motionless in the air, until the twigs return to their feet. When looking out for prey, this Kite usually maintains a height of sixty or seventy feet above the ground, and in its actions strikingly resembles a fishing gull, frequently remaining poised in the air with body motionless and wings rapidly vibrating for fully half a minute at a stretch, after which it flies on or dashes down upon its prey. The nest is placed on the topmost twigs of a tall tree, and is round and neatly built of sticks, rather deep, and lined with dry grass. The eggs are eight in number, nearly spherical, the ground-colour creamy white, densely marked with longitudinal blotches or strips of a fine rich red, almost like coagulated blood in hue. There is, however, great variety in the shades of the red, also in the disposition of the markings, these in some eggs being confluent, so that the whole shell is red. The shell is polished and exceedingly fragile, a rare thing in the eggs of a raptor. 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.)