STRUCTURAL ADAPTATIONS 397 



ground-dwelling forms as Grouse and Sand-grouse have peculi- 

 arly short tarso- metatarsi, differing in this respect from all their 

 allies wherein this segment of leg is long! 



The effect of disuse, or, more correctly, the absence of selec- 

 tion, on the size of the hind-limbs, are nowhere more strikingly 

 illustrated than in the Frigate-birds, Humming-birds and Swifts, 

 All these types spend almost the whole of their waking hours 

 on the wing, and in consequence the hind -limbs are reduced to 

 the smallest possible size consonant with the support of the 

 body. The Frigate-birds have retained the type of swimming 

 foot peculiar to their allies the Gannets and Cormorants — all 

 the toes being united in a common web — though they never 

 dive, and rarely if ever swim ; but the toes of the Swift have 

 undergone some transformation, all the toes pointing forwards 

 {pamprodactylus), whereby they are enabled to cling to the steep 

 faces of cliffs or under eaves of houses while they feed their 

 young. 



The capture of swiftly moving prey, often of large size and 

 of considerable weight, must always, in the case of birds, be 

 effected by the legs. To render this possible considerable 

 transformations in the hind-limb have taken place, though these 

 are mainly confined to the tarso-metatarsus and toes. In the 

 smaller birds of prey the feet only have undergone any marked 

 change. In so far as the skeleton is concerned it has simply 

 increased the size of the ungual or claw-bearing phalanges. 

 As might be expected, all four toes are present ; the hallux, so 

 often reduced or even absent in birds which do not perch, is 

 in birds of prey always strongly developed. In the larger birds 

 of prey, such as the Eagles and the Great Owls, the tarso- 

 metatarsus loses its cylindrical shape and becomes transformed 

 into a shaft flattened and twisted into long and broad ledges. 

 These features are most marked in the great Harpy Eagles 

 iThrasaetus). The grip of the foot is increased by the develop- 

 ment of fleshy bulbs {tylari) of considerable size on the under- 

 surface of the toes, and these are especially long in the Sparrow- 

 hawks. 



The armature of the feet is completed by the development 

 of enormous claws, of which the largest is that borne on the 

 hallux. The strength of the feet of raptorial birds is enormous. 

 The Goshawk, for example, kills its prey by the grip of its feet. 



