Comparative Periods of Nestling Life 7 



meet with this fate . . . those species which, while re- 

 taining their arboreal nesting habits, have adopted the 

 method of curtailing the activity of the young. This pro- 

 cess was accomplished by reducing the food-yolk within the 

 egg, and thus inducing an earlier hatching period. We 

 may approximately measure the extent to which this re- 

 duction has been carried by the degree of helplessness dis- 

 played by the newly hatched bird, and by the nature and 

 extent of its clothing. . . . The amount of food-yolk 

 once reduced, a return to the older fashion of active young 

 was impossible, and this explains why young of many 

 species hatched upon the ground are as helpless as those 

 reared in the topmost boughs of the highest trees." 



Young reptiles are always active' at birth, and there 

 is little doubt that tlie uidifugous or precocious bird is the 

 most primitive type. It is reasonable to suppose that the 

 Arcluropterij.r (a possible progenitor of the l*asseres) was 

 uidifugous, but it cannot be known positively that it was 

 arboreal as a breeder ; in fact its beak seems poorly adapted 

 for nest-building, (few, if any of our Passerines employ the 

 feet to any extent in that capacity) and there are well 

 known instances of arboricole species of Xorth America 

 (the Yellow-bellied Flycatcher, Black and White Warbler 

 and Hermit Thrush, for instance i descending to the ground 

 to nest. 



With accurate knowledge of the condition and early 

 life history of only a limited number of North American 

 birds, it is perhaps somewhat presumptuous to offer serious 

 objection to the Pycroft theories, but my investigations, 

 however superficial and touching only a small angle of the 

 subject, would seem to show here and there an invalid 

 premise or an erroneous conclusion. 



Confirmatory evidence of the theory that birds were 

 originally arboreal, would seem to be lacking in the be- 

 havior of our birds. In the more than occasional depo- 

 sition of the eggs of individuals of species normally 

 ground-nesters (the Herring Gull, some Ducks and Geese) 

 in nests high up in trees, and the habitual arboreal nesting 

 of some species of tyi)ical terrestrial-nesting groups of nidi- 



