78 DWIGHT 



tris) being one of the earliest (early in December), and the Tree 

 Sparrow (Spizella monticola) one of the latest (early in Janu- 

 ary), to complete the ossification. These dates are approximate, 

 but they throw some light on a neglected page of bird study 

 that I now turn for the first time. The late ossification of other 

 bones should be mentioned in passing, but most of them require 

 such careful examination as to preclude their ready use in deter- 

 mining the age of the bird. 



The bearing all this matter has on the question of moult is 

 this : if, when a species departs south in the autumn, we know 

 exactly the plumage of the adult and exactly that of the 

 young bird, it is far easier to interpret the changes that have 

 taken place in each when they return in the spring, for the 

 amount of moult and the amount of wear varies according to 

 age. The new aspect of the plumage may be entirely due to 

 wear, to moult, or to a combination of the two. A method has 

 been suggested for telling old from young in the fall by the 

 presence of sheaths on the primaries in adults and their absence 

 in young birds, because the latter do not usually moult these 

 feathers in assuming fall dress, but it fails both in young birds 

 that do renew the primaries, and in old birds that often show 

 moult elsewhere after the primaries have lost their sheaths. 



Wear or Feather Disintegration 



Some of the effects of this complex process are illustrated on 

 plates I, II, IV, VI and VII, where a change in the shape and 

 color of feathers is produced by loss of substance, generally at 

 their margins. The destructive influences to which feathers are 

 exposed may best be summed up under the word wear, which 

 means a great deal and should be thoroughly understood in 

 studying the relation of plumages and moults. The chief fac- 

 tors concerned in wear are abrasion and fading, which always go 

 hand in hand the one mechanical disintegration, the other chem- 

 ical decoloration, but there are a number of minor factors which 

 modify their effects. The age of a feather, its position, its struc- 

 ture, its color and the habits of the bird, are all matters that 



