398 A HISTORY OF BIRDS 



Where fish form the staple diet the under-surface of the toes 

 lack the tylari just described, and instead have the skin beset 

 by sharp denticulate horny spines, and this feature has been 

 acquired independently by the Ospreys on the one hand and 

 the Fishing Owls {Ketitpa) on the other. This roughened 

 surface obviously is of the highest importance in maintaining 

 a grip on slippery creatures such as fish. 



In so far as structure is concerned the feathers change but 

 little in response to environment. Those of aquatic birds differ 

 but little from those of birds which never or rarely enter the 

 water, and when this is said there is but little else to say. Their 

 function is to subserve the purposes of flight, and of a covering. 

 But in some cases the tail feathers are utilised to serve as the 

 third leg of a tripod, and this obtains in the Penguins and the 

 Woodpeckers, and one or two other climbing birds. To this 

 end these feathers have acquired a remarkable stiffness and 

 have the shafts pointed at the tips. By this means the Wood- 

 pecker is enabled to support its body when clinging to tree- 

 trunks, or when driving holes therein for nesting purposes, or 

 in search of food. At this time these long elastic spines are 

 of the greatest service, affording the bird a leverage when deal- 

 ing the series of powerful blows necessary to attain its ends. 

 But it is a curious fact that the Nuthatches, which are as skilled 

 at this work as the Woodpeckers, and as rarely leave the trunks 

 of trees, have not acquired this peculiar form of tail, nor have 

 they, by the way, developed the same modification of the foot 

 as obtains in the Woodpeckers. Similarly, the aberrant Wry- 

 necks, which agree with the Woodpeckers in having an elongate 

 protrusible tongue and zygodactyle feet, have comparatively soft 

 tail feathers. This is not to be explained by the fact that the 

 Wryneck does not use its beak as a hammer, since the little 

 Tree-creepers, which like the Nuthatches are true Passerines and 

 therefore unrelated to the Woodpeckers, have developed spiny 

 tails, yet they have slender curved beaks, and are therefore in- 

 capable of using the beak after the fashion of the Woodpecker. 

 This is all the more puzzling since certain South American 

 Passerine types, known as the Woodhewers {Dendrocolaptincu) 

 have in many species adopted the habits, and to a large extent 

 the outward form, of the Woodpeckers, especially in regard to 

 the spiny tail. 



