422 A HISTORY OF BIRDS 



The Parrots, we might mention, are the only birds which 

 universally hold their food in the foot while it is being eaten ; 

 and it >vill be found, moreover, that with certain exceptions the 

 left foot is invariably used for this purpose. The little Owl 

 {Athene noctud) is perhaps the only other bird of similar habits. 

 The Hobby among the Accipitres uses the feet for a similar 

 purpose, but only when in mid-air. 



There is a remarkable diversity in the shape of the beak 

 among the fruit-eating birds. In the case of the Parrots hard 

 seeds and nuts form the staple diet ; the exceptions to this rule 

 are obviously recent changes of habit which have not yet greatly 

 affected the bill. In the Toucans and Hornbills it is of enormous 

 size, so much so as to give these birds the appearance of being 

 terribly overweighed by their bills. As a matter of fact, how- 

 ever, these enormous jaws are really of astonishing lightness, 

 especially in the Toucans. A section through the bill of one of 

 these birds shows that the horny beak-sheath is extremely thin, 

 and covers a lattice-work of bone of extreme delicacy, resemb- 

 ling filigree work, the interstices of which are filled with air. 



While the bill of the Toucan is remarkable for the brilliancy 

 of its colours that of the Hornbill is scarcely less so for its 

 bizarre ornamentation, which takes the form of a casque of 

 varying shape. In some species this casque is furthermore 

 peculiar in that it is open in front, giving a strangely unfinished 

 appearance to this ornament. What is still more strange is the 

 fact that this opening leads directly into a mass of cancellated 

 tissue. The casque of the Helmet- Hornbill {Rkinoplax vigil) 

 presents something of a mystery, inasmuch as it is com- 

 posed of a mass of ivory-like density, and of great thickness. 

 What can be the use of this structure ? It has been suggested 

 that the bird uses it as a hammer for breaking open hard- 

 shelled nuts, and certainly it is a fact that the face of this mass is 

 bruised as if it were put to some such use. Further, the fossae 

 or cavities which lodge the cerebral hemispheres of the brain, 

 and of the optic lobes, and the septum which dips down be- 

 tween the two cerebral hemispheres in the middle line are un- 

 usually thickened, as though to protect the brain from the 

 effects of violent shocks. To deaden the force of such shocks 

 the horny mass is backed by a bony network of props directed 

 forwards and meeting its base at right angles. These mechani- 



