Though the skull is small in _Circus_ and some other forms, it is usually large and broad, being considerably elongated in the Aquilinae. The short stout bill is strongly curved, and terminates in a hook, which is often nearly perpendicular, and is specially prominent in _Rostrhamus_, _Leptodon_, _Harpyhaliaëtus_, _Pithecophaga_ and _Thrasaëtus_; the basal third is straight in Eagles, while the edges of the maxilla are lobed or festooned to a variable extent, and in the Falconinae are distinctly toothed, or even bidentate in the case of _Spiziapteryx_, _Harpagus_, and _Baza_. A bony ridge over the eye conduces to the fierce aspect, especially in the larger species. The feet are robust and well-fitted for grasping, and are enormously developed in _Thrasaëtus_; the metatarsus is much flattened, and may be scutellated or reticulated, though the scales are usually smaller behind; the tibia generally exceeds it in measurement, but in Accipitrine forms is nearly equal, giving them a long-legged appearance. Elongated bare metatarsi are characteristic of _Circus_, _Polyboroïdes_, and the Polyborinae. The claws are sharp and curved, especially in _Rostrhamus_; a short membrane connects the middle and outer toes, and the inner also in the Polyborinae; while their under surface is more or less padded, and exhibits rugose spicules below in _Busarellus_, similar to those in _Pandion_. The powerful wings may be long and pointed, as in the Kites, Falcons, and Harriers; moderate and somewhat rounded, as in the Eagles and Buzzards; or short and narrow, as in Hawks. Falconers term the long-winged forms "noble," the short-winged ignoble. The tail, usually of medium size, but sometimes very short, as in _Helotarsus_ and _Gypohierax_, is decidedly elongated in the Accipitrinae and _Polyborus_, and also in _Milvus_, _Lophoictinia_, _Elanoïdes_, and _Nauclerus_, where it is forked–very deeply in the two last: it may be wedge-shaped, as in _Uroaëtus_, _Thalassaëtus_, _Harpyhaliaëtus_, and _Gypaëtus_; rounded, as in _Elanus_ and _Haliaëtus_; nearly even, as in _Buteo_ and _Aquila_; or emarginated, as in _Ictinia_ and _Rostrhamus_. Normally there are twelve rectrices, but _Thalassaëtus_ has fourteen. The colour varies greatly with age, {147}and it often takes four years or more to attain maturity, the markings commonly changing from longitudinal to transverse; but the sexes are usually alike, though the Kestrel, Merlin, Red-footed Falcon, and many Harriers are well-known exceptions, the last having generally blue-grey males and brown females. The occipital feathers are elongated in several of the Polyborinae, and a full crest occurs in many genera, _Lophoaëtus_, _Thrasaëtus_, _Harpyhaliaëtus_, _Helotarsus_, _Morphnus_, and _Lophoictinia_ being especially noticeable; _Circus_ has a facial ruff, coupled with exceptionally large aural apertures; the feathers of the neck may be lanceolate, as in _Haliaëtus_, or those of the nape, as in _Aquila_;