92 THE WILSON BULLETIN— June, 1921 



further comparisons. Considering size, the Hummingbird 

 group appears to exceed all other groups, botli in duration 

 of incubation and the length of time tlie young remain in 

 the nest : ai)proaching like periods of tlie much larger- 

 bodied and closely allied Swift. 



Second. The normal nestling period of any species equals 

 or exceeds the period of embryological development; i.e. 

 when the period of incubation of a species is brief, the time 

 in which the young occupy the nest is correspondingly 

 brief and when the incubation of a species is of long dura- 

 tion the 3'oung are a long time helpless. Fright or rest- 

 lessness may drive some species prematurely from tlie nest, 

 especially ground or near ground nestling species like the 

 Pelican, Cormorant, Ibis, Bittern, Heron, Dove, Vulture, 

 Marsh Hawk, Short-eared Owl, Cuckoo, Horned Lark, Bob- 

 olink, Meadowlark, and many of the Sparrows, Warblers, 

 Mockers and Thi-uslies, but they seldom travel far from the 

 nest for some time. 



Third. Young born naked, never acquiring natal down 

 (gymnopcedic, or psilopcedic) including the Parrots, 

 Cuckoos, Woodpeckers, Kingfishers, Hummingbirds and 

 Swifts, develop slower physically than most nidicolous 

 birds born with or acquiring a full or scanty clothing of 

 natal down ; though this is not so apparent in the Ameri- 

 can Cuckoos, since the early development of the feet of 

 these birds enables them to clamber about at an early age ; 

 an important qualification for the perservation of the 

 young of these large, bush-nesting birds. The European 

 Cuckoo, however, vacates the nest only a few days before 

 flight which occurs when about twenty-one days old {Gf. 

 Taylor, The strand Magazine, xli, pp. 652-656). It is 

 noteworthy tliat the nestlings of most other species liabitu- 

 ally breeding in situations affording extraordinary pro- 

 tection from natural enemies and the weather, or of species 

 nesting in less protected situations but capable of defend- 

 ing the young from any ordinary danger, have young that 

 are rather slow to depart from their nests and their de- 

 parture signifies the completion of their fledgling, as well 

 as nestling life. The impulse to vacate is not ordinarily 



