Comparative Periods of Nestling Life 5 



ditiou of the young at birth as a taxoiioinic character, 

 though in the past, unsuccessful efforts have been made to 

 produce a satisfactory physiological arrangement of our 

 birds based upon this peculiar state, grouped in accordance 

 with external characters such as the shape of the bill and 

 feet. 



In the Water Birds there are some groups of NidicohT 

 more nearly related in every essential character to the 

 lower Kidifuga?, though the relatively higher group, the 

 strictly Land Birds, are nearly all NidicoliB, tlie exceptions 

 being the Gallinaceous birds. 



Nidificate species constructing elaborate nests or labor- 

 iously tunneling in the earth or wood, are all or nearly all 

 nidicolous ; while the nidifugous birds are essentially ter- 

 restrial and frequently deposit their eggs on the bare 

 ground or rocks, with the exception of the Ducks, which 

 heavily line their nests with feathers plucked from their 

 own breasts; at best their nests are little more than 

 rubbish heaps of earth, leaves or grasses. As a taxonomic 

 character for groups, there are some inconsistencies; the 

 Alcidfc, for instance, are not all nidicolous; according to 

 recent observations the young of the Murrelets (at least 

 the Ancient, Xantus's, and Craveri's Murrelets, all of 

 which take to the sea in from one to three or four daj's ) , 

 are nidifugous. 



The higher types of Nidicoloe (young hatched in a 

 blind or helpless and naked or semi-naked condition, never 

 acquiring natal down, or acquiring natal down growing 

 from the tips of the juvenal plumage) are characteristic of 

 the higher, more specialized groups. 



It has been thoughthiot only that the ancestral type of 

 our birds was nidifugous but that the protective aboreal 

 nesting habits led to nidicolous young; a hypothesis not at 

 all nullified by the fact of so many nidicolous species be- 

 longing to phylogenetically older groups and to groups 

 having ground-nesting or near ground-nesting habits; for 

 these species nest exclusively or almost exclusively in col- 

 onies and on islands affording protection from their natural 

 enemies equal if not superior to the elevation of the arbor- 



