Comparative Periods of Nestling Life 9 



sought an elevated situation to avoid molestation. In the 

 AJciiur, perhaps several species are in an active state of 

 evolution from typical Nidifngiie to a condition of nidicol- 

 ons habits. The great bulk of the Atlantic, California and 

 Brnunich's Mnrres, and the Kazor-billed Ank, tirst see light 

 on narrow ledges far above the sea in almost inaccessable 

 cliffs, positions doubtless chosen ages past by the species 

 to avoid molestation ; the result is the enforced helpless- 

 ness of the young for a period of from 25 to 35 or 40 days, 

 until half grown ; though normally possessed of consider- 

 able strength and activity when only a few days old. 

 Macgillivray gives instances of the extraordinary hardi- 

 hood of some very small, unfledged young of the ^lurre, 

 which Avere observed swimming about in the vicinity of the 

 rocks ; chicks that must have had comparatively eas}' access 

 to the Avater. 



There are no data from which to buihl a table of com- 

 parative values of the yolk in birds' eggs, but my study of 

 the incubation periods of many North American birds, 

 convinces me that the eggs of the Nidicolpe do not always 

 show a relatively earlier hatching period in comparison to 

 the Nidifugjp. Eggs of some species vary enormously in 

 size. I have the shells of some eggs laid by a small domes- 

 tic hen, averaging 2.92 x 2.2f) in inches, comparable to the 

 measurements of the eggs of some of the Megapodes. Some 

 typical Nidifugne lay eggs of normal size and it is probable 

 that the yolk is pro])ortionally as small as many of the Xidi- 

 colfP. It is possible that the egg of the Hummingbird con- 

 tains yolk comparable in bulk proportional to that of the 

 Murre (both species laying eggs of very large size in com- 

 parison to the bodies of the parents) yet there are none 

 more naked and lielpless than the newly hatched young of 

 the former, while the nidifugous young of the latter would 

 buffet the sea in a few days were they not literally' shelved. 



Is a return to nidifugous habits im[)0ssible? AVho can 

 say in the instance of the Kighthawk and Whip-poor-will, 

 whose young are enabled to abandon the nesting site on the 

 ground within a fcAv hours after birth, whether they are 

 not approaching typical nidifugous habits in the early use 



