BIRDS 



453 



problem of separating the pure and impure blood in the circu- 

 lation, a feat which reptiles have not yet accomplished. Hence 

 the hot-bloodedness of birds, and the evolution of the incubatory 

 habit. 



Some birds make no nest at all, but simply lay their eggs in 

 situations which are more or less exposed. The Common Guillemot 

 ( Uria troile], for example, lays her single egg on a ledge of rock 

 or in a crevice, situated on a cliff or stack, the inaccessibility of 

 which is a sufficient protection from most enemies, man excepted. 

 The Common Tern (Sterna fluviatilis^ lays two or three mottled 

 eggs among shingle (fig. 970), 

 and it will be seen from the illus- 

 tration that it is scarcely possible 

 to speak of a nest. The eggs 

 are protectively coloured, as is 

 usually the case when they are 

 exposed to view, and by no 

 means easy to detect. The Ken- 

 tish Plover (/Egialitis Cantiana] 

 affords another illustration of the 

 same thing. Dixon (in Among 

 the Birds in Northern Shires] 

 says of the Nightjar or Goat- 



sucker ( CaprimulgUS EurOpceUS) '. 



"It makes no nest, but the 



hen bird lays her two curiously oval eggs on the bare ground, 

 sometimes beneath a spray of bracken or a furze bush, less 

 frequently on the flat low branch of a convenient tree. These 

 eggs are very beautiful, and he who finds them cannot confuse 

 them with those of any other species that breeds in our islands. 

 They are generally white and glossy, the surface mottled, blotched, 

 streaked and veined with various shades of brown and gray." 



Many birds make a burrow or hole in which to nest, or take 

 advantage of an existing cavity, and the eggs are white, which 

 is the general rule where they are not exposed to view. The 

 Sand Martin (Cotile riparia) tunnels out a long (up to nine feet) 

 and fairly horizontal gallery on the side of a steep bank often 

 overlooking a stream. The inner end of this is enlarged, and the 

 four to six eggs laid upon a layer of grass and neatly-arranged 

 feathers. The Kingfisher (Alcedo ispida) digs out an upwardly 



F5g 970 .-E g gs of the Common Tern (Sterna 



