432 THE BIRDS xvi. z- 



and varies from 200 per 10 acres in woodland to 20 on agricultural land 

 and 10 or less on moorland. Calculating from such figures Fisher 

 estimates that there may be 100 million land birds in Great Britain 

 and 100,000 million birds in the world altogether, including sea birds. 

 This is perhaps a low estimate ; it would represent a total biomass of 

 the same order as that of 3,000 million human beings. With all their 

 activity, therefore, the birds organize less matter into themselves than 

 do the mammals. 



One of the most striking features of bird life is that although the 

 basic organization remains fairly constant differing types show a great 

 variety of special features, fitting them for numerous habitats. Besides 

 differences in behaviour, in body form, and in powers of flight there 

 are found others in the shape of the bill, and hence of food habits, and 

 in the details of many other parts, such as the feet, that make fascinat- 

 ing studies in adaptation to environment. 



3. The skin and feathers 



The skin of birds differs from that of mammals in being thin, loose, 

 and dry; there are no sweat glands, indeed the only cutaneous gland 

 present is the uropygial gland or preen gland at the base of the tail. 

 The bird cleans its feathers with its beak, obtaining oil from this gland, 

 which is especially well developed in aquatic birds. 



The keratin-producing powers of the skin are of course mostly 

 devoted to producing feathers, but scales like those of reptiles are 

 present on the legs and feet and sometimes elsewhere. The bill (p. 466) 

 and claws are also specialized scale-like structures and are sometimes 

 moulted. 



Nerve-endings are present throughout the skin, and the cere at the 

 base of the bill is perhaps an organ of touch. The bill may itself have 

 special endings, such as the corpuscles of Grandry found in the ducks. 



The feathers of modern birds provide a covering whose uses vary 

 from heat insulation and flight to protective coloration and sexual 

 display. It is likely that in evolutionary history the function of heat 

 regulation came first. The two main functions of heat conservation and 

 flight are indeed today performed by feathers of different types (Fig. 

 249). The down-feathers or plumules, which form the covering of the 

 nestling and may be present also in the adult, are simpler than the 

 contour feathers or pennae, and the elaborate flattened flight feathers. 

 Filoplumes are a third type, being very fine, hair-like feathers. Usually 

 several generations of feathers are produced ; first the nestling feathers 

 (neoptiles), then one or more generations of juvenile feathers, which 



