NESTLING BIRDS AND WHAT THEY TEACH 247 



The number of species which have adopted this expedient 

 outnumber those which have not, and this speaks volumes for 

 its success. As examples we may cite the vast army of song- 

 birds, the Pigeons, Parrots, Cuckoos and birds of prey. 



The amount of food-yolk once reduced, return to the older 

 fashion of active young became impossible ; and this explains 

 why the young of so many species hatched on the ground are 

 as helpless as those reared in the topmost boughs of the highest 

 trees. These young are the descendants of birds whose young 

 had become adapted to the requirement of an arboreal nursery, 

 and though this was later forsaken it is obviously impossible to 

 return to the earlier precocious condition. 



The young of the Skylarks, Pipits and Wagtails, among the 

 Passeres, and of the Bee-eaters, Kingfishers and Night-jars 

 among the Coraciiformes (p. 59) are cases in point. Though 

 these are now hatched in a nest on the ground, or as in the case 

 of many Kingfishers and of Bee-eaters, in burrows, they have 

 all undoubtedly undergone this adaptation to the needs of an 

 arboreal nursery. The factors at work which determined the 

 change in the nesting site are discussed elsewhere. 



On the other hand, there are a number of instances where 

 precocious young, specially adapted to meet the requirements 

 of a terrestrial nursery, are reared in trees, as in the cases of 

 the young of the Green Sandpiper {Totanus ochropus) and of the 

 Noddy {Anous stolidus) and White Tern {Gygis Candida). The 

 condition of these nestlings leaves no room for doubt that they 

 have only lately ceased to become actually precocious ; in form 

 they are still scarcely distinguishable from the still active young 

 of their immediate allies. They are apparently on the way to 

 become as helpless as the young of the Gannets, Cormorants, 

 Pelicans and their allies, which, in the early stages of develop- 

 ment are now peculiarly helpless. This stage has probably 

 been reached by a series of gradual transitions similar to those 

 now in progress by the Terns just described. Like these birds 

 and the Green Sandpiper, the Gannets and Cormorants, Frigate- 

 birds and Pelicans, now, either sporadically, or in the case of 

 some species, constantly nest in trees. Such a nesting-place, 

 doubtless, has only lately been resorted to — it is a reversion 

 to an ancient custom and not a survival, as in the case of the 

 Hoatzin. 



