and the plumage commonly over-hangs the metatarsus, which is feathered to the toes in various Aquiline forms, and in _Archibuteo_. The nostrils are circular in the Falconinae, oval or nearly linear elsewhere, with a central tubercle in the last-named and the Polyborinae, seldom found in the other Sub-families: they are generally in or near the cere, which is almost always fleshy. An aftershaft is present; the down in adults is uniform; that of the nestling being woolly and varying from white to grey, buff, brown, or black. The feet are yellow, red, or brown; the bill is ordinarily dark, and the cere yellow; _Gypaëtus_, however, has all these parts bluish-grey, with a crimson sclerotic membrane (equivalent to the "white of the eye") round the orange iris, the latter being yellow or orange in the Accipitrinae, brown in the Falconinae, and varying to red elsewhere. The syrinx has two pairs of tracheo-bronchial muscles; the tongue is thick and often concave; and Nitzsch[132] has recorded single or paired powder-down patches on the lower back of _Elanoïdes_, _Elanus_, _Regerhinus_, and _Circus_, with similar but scattered down-feathers in _Gypaëtus_. The members of this Family range in size from the mighty Lämmergeier to the tiny Finch-Falcon (_Microhierax_); but they have many habits in common, though _Polyborus_ and _Milvago_ are somewhat terrestrial and vulturine, and a few species have crepuscular tendencies. They are decidedly non-gregarious, though the Polyborinae, _Erythropus_, and _Rostrhamus_ form partial exceptions; they pair very early in the year, if not for life, the larger forms in especial breeding almost before winter is over. Birds of the mountains, the plains, and the woods, they can bear the cold of the icy regions or the heat of the Equator, but towards {148}either pole the number of species decreases perceptibly. The sight is exceptionally keen, and the flight generally powerful and rapid; Eagles and Buzzards indeed move heavily to all appearance, as they circle or sail around with flapping action, but the spectacle of the former in chase of a grouse will quickly disillusion the observer. Kites are still more versatile upon the wing, nor are the Polyborine forms always deficient in this respect, while the dash and speed of Hawks and Falcons in their different styles is proverbial.[133] Harriers and the like may be seen buoyantly quartering the ground for hours, poising themselves almost motionless aloft, or gliding in circles to great heights; and the hovering or stationary position on the wing, which gives the name of "Windhover" to the Kestrel, is more general than might be supposed throughout the Family. Taken as a class, few birds can fly so well or so untiringly, though Vultures, Cranes, Storks, Albatroses, and the larger Gulls have even greater powers of endurance; they can, moreover, perch with great facility, and, while seldom running or walking fast, can move with freedom upon the ground, where they generally