236 A HISTORY OF BIRDS 



intimately related. Thus the young of the Ostrich tribe, the. 

 Game-birds, Gulls, Plovers, Sand-grouse, Bustards, Cranes and 

 Rails, Geese, Swans and Ducks, Grebes and Divers, are all 

 precocious (nidifugous). On the other hand, the Penguins, 

 Gannets and Cormorants, and their allies, the Petrel tribe, 

 Hawks, Pigeons, Owls, Cuckoos, Hornbills, Swifts and Wood- 

 peckers, for example, and the Passeres, are all born helpless 

 (nidicolous). 



But no hard and fast line between nidifugous and nidi- 

 colous birds can be drawn in the first place, and as we shall 

 show presently these two states are really due to adaptation 

 and have no genetic connection. 



In the matter of clothing the helpless forms present every 

 possible gradation from a complete investment of down-like 

 feathers to absolute nakedness. Some are hatched naked and 

 later acquire a downy covering, others remain naked until the 

 feathers appear ; and in this no sort of order seems to be 

 traceable. In the precocious types the downy covering of the 

 nestling seems to have attained its full growth at hatching. 



Among the helpless forms this appears to be but rarely, 

 if ever, the case. Young Hawks and Owls, for example, at 

 hatching are but scantily clothed. In a very short time, how- 

 ever, an abundant crop of long and very soft down feathers is 

 developed. So too with young Penguins. The young Pelican 

 and Cormorant leave the shell quite naked, and never develop 

 more than short, downy tufts which barely conceal the skin. 

 The young Gannet differs in this respect inasmuch as on leav- 

 ing the egg the body is covered by an exceedingly short down, 

 which rapidly acquires considerable length. The down of the 

 young of the Petrels, and especially of the Albatross, grows to 

 a great length. The young of the closely allied Storks and 

 Herons differ markedly in the length of the down feathers, that 

 of the Heron being of a peculiarly hair-like nature, whilst the 

 Stork's covering is of the more typical kind — short and woolly. 

 The down of the nestling Pigeon is peculiarly thread-like in 

 appearance, and sparsely distributed. The Kingfishers, Horn- 

 bills, Swifts and Humming-birds never develop nestling down, 

 so that the growing feathers give these birds somewhat the 

 appearance of Hedgehogs. The young of many Passeres are 

 aggressively naked, others develop an apology for a covering, in 



