458 W. P. Pycraft: 



pendently adopted by so many unrelated forms is an interesting 

 and instructive one. 



The amount of food-yolk, however, once having* been 

 reduced, return to the older fashion of nidifugous young 

 became impossible : and this explains why nidifugous young 

 are still born to those parents which have adopted the 

 practice of depositing their eggs upon the ground. It proves 

 that the arboreal habit has been forsaken since the specializa- 

 tion. Some, like the Cormorants, Herons, and certain of the 

 Gull tribe, for example, build, as occasion demands, either on 

 the ground or in trees . Now it is interesting to note that among 

 these birds the young Cormorants and Herons are completely 

 nidicolous, the Gulls only partially so, whilst the near and 

 less specialized allies of the Gulls, the Plovers, have nidi- 

 fugous young. The nidicolous condition of the Cormorants and 

 Herons must undoubtedly be regarded as the result of adapta- 

 tion to an arboreal breeding site, as in the case of the Song- 

 birds for example, whilst the peculiar intermediate condition 

 of the Gulls seems to show that these are really nidifugous 

 forms whose activity is being restricted, as in the case of the 

 arboreal types, but for other reasons to be discussed forthwith. 



To species breeding in large colonies, or on ledg'es of 

 precipitous cliffs, the reduction of the food-yolk and conse- 

 quent helplessness of the young are obviously advantageous ; 

 for in the former case large numbers of young, if nidifugous, 

 would go unfed and soon starve owing to the impossibility 

 of the parents recognising them running about amongst 

 their neighbours, whilst in the hitter case, among the young 

 of cliff-breeding species, great activity would be accompanied 

 by an enormous mortality owing to falls from the cliff. 



In conclusion we may say a few words about the young of 

 the Megapodes. The eggs of the Megapode are, as is well 

 known, hatched in decaying vegetable heaps., or in hot sand, 

 instead of being incubated by the parent. To this end the 

 amount of food-yolk has been enormously increased, thus 

 enabling the normal nestling period to be passed within the 

 egg, the young passing through the downy stage during em- 

 bryonic life, and emerging from the shell fully fledged, and able 

 to fly. Thus the means sought to secure the safety of the young 

 is exactly opposite to that adopted by arboreal breeding birds. 



