YOUNG BIRDS AND RECORDS OF THE PAST 115 



young crows are, at hatching, in a perfectly helpless 

 condition, quite naked, and with the eyelids still sealed. 

 They are " prematurely born," so to speak. On the 

 other hand, young chickens and young ducks come into 

 the world in a much more advanced condition, having 

 the body thickly clothed in down, and being able to run 

 about with their parents within an hour or so of their 

 entry into life. It is plain that this advanced condition 

 is due to the fact that, owing to the larger amount of food 

 yolk enclosed within the shell, development has proceeded 

 further before hatching ; but the reasons for these differ- 

 ences in development and the relative activity of the two 

 types of nestling are not so clear. 



Ornithologists have never seriously set themselves the 

 task of interpreting such differences, but have been content 

 to label the helpless type a " nidicolous " or " altricial " 

 nestling, the active a " nidifugous " or " precocious " 

 nestling. And having duly labelled them, they have next 

 proceeded to make use of these two types as a basis of 

 classification. 



Roughly speaking, we may regard the young of the 

 ostrich tribe, the game-birds, gulls, plovers, sand-grouse, 

 bustards, cranes, rails, geese, swans, ducks, grebes, and 

 divers as nidifugous or precocious types : while those 

 of the penguins, gannets, and cormorants, and their allies, 

 the petrel tribe, hawks, pigeons, owls, cuckoos, hornbills, 

 swifts, woodpeckers, and the Passeres or " perching " 

 birds, e.g. crow and thrush, are altricial or nidicolous. 

 But let this fact not be forgotten — no hard-and-fast line 

 can be drawn, between the precocious and helpless types, 

 and for reasons which will appear presently. 



In the matter of clothing the helpless forms present 

 every possible gradation, from a complete investment of 



