HOW SOME BABY BIRDS ARE FED. 131 
of the fish between the place of capture and the 
nest. 
In this respect the petrels go further and convert 
the fish into an oily substance which is ejected for 
the young. The baby petrel revels in the delights of 
a cod-liver-oil diet from the start. 
The Pigeon group is very peculiar even among 
this kind of birds, in that the young inserts its beak 
into that of the parent and finds there at first not 
half-digested food but a curdlike secretion, or, rather 
more accurately, the thickened and “ peeled up” lin- 
ing of the parent’s crop. Until the young are about 
nine days’ old this occurs in both parents, as an un- 
explained physiological result of incubation. Toward 
the last of this period this curd is mixed largely with 
the food of the parent, and gradually ceases to form 
till the youngster finds for his dinner only bread 
without cheese. 
Only among the hornbills perhaps is there found 
anything similar. Here the male at the nesting sea- 
son walls his mate into a hollow tree, plastering up 
all the opening except a small hole. Through this 
he brings her—and in due time her single nestling— 
food, which is usually fruit. This is ejected from his 
stomach—not the crop. In casting it up the entire 
mass is inclosed in a gelatinous envelope or pellicle, 
which is a temporary lining of his stomach. The 
wrapper goes along with the goods, and the little bird 
and his mother have long antedated us in taking things 
in capsules. This is a very convenient arrangement. 
Even in the higher birds this regurgitating habit pre- 
