STRUCTURAL ADAPTATIONS 393 



strictly arboreal life are few. Such structural modifications as 

 we meet with chiefly affect the feet. Furthermore, as one 

 would expect, the birds so affected are those which spend 

 practically their whole lives in trees or amid scrub. 



Such species as resort to trees merely for nesting purposes 

 not only undergo no adaptation to facilitate perching, but they 

 succeed in maintaining a secure foothold with feet which have 

 completely lost one of the principal agents for grasping purposes 

 — the hind toe. That is to say, their feet have become pro- 

 foundly modified in adaptation to totally different needs, show- 

 ing that they have returned to such nesting or roosting resorts 

 after having forsaken them for countless generations. 



Such abnormal perchers are, so to speak, like fish out of 

 water, but they are extremely instructive types, and show that 

 the greatest caution must be exercised in generalising on the 

 habits even of the most specialised types. Thus Gannets, 

 Cormorants, Frigate-birds, Ducks and Terns are scarcely birds 

 one would expect to find roosting and even nesting in trees ; 

 not because, judging from the habits of the majority of the 

 species belonging to these groups, such roosting or nesting- 

 places would never be selected, so much as from the fact that 

 their structural peculiarities would seem to render them utterly 

 unfit to use such retreats. 



Thus the Gannets and Cormorants are typically cliff-haunt- 

 ing birds, laying their eggs on ledges, or on the ground, yet 

 several species of Gannets and of Cormorants build in trees : 

 the Darter invariably does so. Among the Ducks the same 

 obtains, and so too with the Gulls, as we have already shown. 

 It is probable that this peculiarity owes its origin to the lack 

 of suitable nesting-places near enough to the feeding-grounds, 

 though to-day it is practised when no such reasons obtain. 

 These birds, in short, maintain their hold on the environment 

 in spite of their manifest unfitness for it. 



With the birds of prey, which are no less highly specialised, 

 perching is easy: their powerful feet have become adapted to 

 grasping prey, and can as easily therefore grasp a bough. 

 These, then, though tree dwellers to a certain extent, have 

 developed no adaptation to this end. 



The feet of that great army of birds known as the Passeres 

 or perching-birds, show the results of a compromise between 



