62 THE BIOLOGY OF BIRDS 



swimming stroke of the hind leg. It occurred also in 

 strong development in the extinct toothed Hesperornis, a 

 sort of giant loon, of the Cretaceous period. Now it is an 

 interesting technical point that this knee-process, which has 

 the same appearance and the same functional significance 

 wherever it occurs, is structurally of three different types. 

 (a) It may be a huge patella fused to the upper end of the 

 tibia, which shows only a small cnemial crest. This is the 

 case in Hesperornis. (b) It may be a prolongation of the 

 cnemial crest of the tibia which has fused with a large 

 patella. This is the case in Grebes, (c) It may be an 

 exaggerated cnemial crest with a mere scale of a patella. 

 This is the case in Divers. 



§ 4. Adaptations of the Skull 



Much follows from the fact that the bird has surrendered 

 its hand in the making of the wing. This involves a utilisa- 

 tion of the skull in a notable way as a manifold instrument, 

 e.g. for seizing the food, for reaching to the ground, for 

 hanging on by, for manipulating the nest materials, and for 

 preening the feathers. In some cases, of course, the feet 

 have taken on hand duties, as when the osprey seizes the 

 trout in its talons. What cranial peculiarities can be 

 interpreted as adaptive ? 



(a) The early fusion of most of the bones in Flying 

 Birds, a fusion so thorough that boundaries are often quite 

 obliterated, makes the skull more of a unified instrument. 

 This may often be an advantage, as in the case of the wood- 

 pecker which uses its skull as a hammer. The movable 

 bones of the bird's skull are the lower jaw, the quadrate on 

 which it works, the rod-like columella running from the 

 drum of the ear to the inner ear, and the hyoid supporting 

 the muscles of the tongue. But the pterygoids on the roof 

 of the mouth usually articulate freely with the basisphenoid ; 

 the lachrymals in front of the orbit may remain free ; and 

 there is often a flexible region — almost a joint in parrots — 

 where the beak joins the frontals. It is interesting to 



