BIEDS. 



51 



(chalazce), — finally of protective structures : the shell- 

 membrane (e) and the calcareous shell (f) ; g is the 

 air-chamber. 



When the young escape from the egg, they are 

 either able to at once look after themselves more or 

 less, at least to look for their food, — in which case 

 they can see and are clothed with feathers at hatch- 

 ing (precocious young; e.g. fowls, ducks, gulls, and 

 pewits), — or the young remain some time in the nest, 

 as they are, to begin with, both blind and naked, and in 

 this case they are fed for some time by the parents 

 {nestlings; e.g. birds of prey, sparrows, nightingales, 



Fig. 29.— a Bird's Egg. 



pigeons). In the frigid and temperate zones most 

 species of birds do not remain in their native country 

 after the breeding season ; those which go south in 

 the autumn are termed migrants ; while birds which 

 do not migrate, but remain in the district where they 

 have bred, are known as residents (sparrow, jay, 

 magpie). Oipsy migrants execute more or less ex- 

 tensive wanderings, influenced by want of food or 

 other causes (woodpecker, titmouse, golden-crested 

 wren, tree-creeper). The travels of such birds are 

 not, like those of migrants, undertaken at a definite 



