CAUSE OF PRODUCTION OF "DOWN." 165 



1. The whole source of food-supply for the young bird is now 

 changed. Heretofore, as an embryo it has formed its tissues 

 from substances once assimilated by the mother-bird and stored 

 by her within the egg, from which it extracts them by means of 

 the yolk-sac, — a provisional appendage of the gut ; henceforth 

 it is subject to the vicissitudes of a much greater and very com- 

 petitive world for its food-supply ; and its entire alimentary tract, 

 with its various appendages, new and untried as it all is, must 

 now begin to work — and work properly and successfully — on 

 the hodge-podge of digestibles and indigestibles which here begin 

 their intermittent flow into it. If there is ever such a thing as a 

 "critical period" in a bird's life, indeed it is here ! We should 

 have the strangest of miracles performed before our very eyes if 

 this transformation and adjustment were to occur instantaneously 

 and without interruption of any of the nutritive processes of the 

 animal. 



2. The skin of the bird is exposed to the chilling and evaporat- 

 ing effects of the air ; this doubtless lessens the blood-supply to 

 the integumentary structures. This chilling, moreover, is now 

 of all times the most effective, for, the bird now has the least 

 plumage to help it retain its heat ; then, too, the heat radiating 

 surface — the skin — is greater in proportion to the mass of the 

 animal than it will ever be again, and therefore the heat loss at 

 that time is greatest. It is, of course, true that such an organism 

 tends to make good the greater heat loss by increased heat pro- 

 duction ; but this latter process means at the same time, a greater 

 use and destruction of food at a time when, as stated above, the 

 nutritive mechanism of the animal has not got into full swing. 



We are, however, in possession of some direct evidence that a 

 faulty nutrition is the cause of the production of the "down." 



1. If a chick is kept continuously underfed from the time of 

 hatching and while in its downy plumage, it will be found that 

 almost all of the feathers (except primaries, secondaries, and a few 

 others) can be kept in the " downy " condition and the bird can 

 thus be made to wear its downy plumage for months (many kept 

 four to five months). 



The " quill " region is a part of the feather which " normally " 

 almost refuses to grow ; by reducing the food-supply during and 



