xvi INTRODUCTION. 
The recent more rapid progress of this science towards 
maturity, may be, in a very great degree, attributed to the at- 
tention paid by some of the later ornithologists to a point which 
had been before almost totally neglected, viz. the changes of 
plumage that the feathered tribe undergo in their progress 
from the young to the adult state, as well as those of a more 
peculiar and partial nature that are experienced at a certain 
season of the year,.sometimes by both sexes, but more com- 
monly only by the male bird. No opportunity has been 
omitted by the present writer to verify (and frequently from 
the progress of experimental observation) many of the changes 
recorded by TEMMincK, Montacu, and others; and, in or- 
der to their elucidation, figures are given of some species at 
different ages, and at different seasons, which will be-more 
apparent in the second part of this work, as these changes 
chiefly exist, and are most striking, in the winter birds; and 
have accordingly been more confusing in their consequences. 
A separate figure also has been given wherever a very marked — 
difference exists between the two sexes; and where this mea- 
sure has not been adopted with respect to the variations m- 
cident to season, they have still been studiously pointed out 
in the description of the species. 
In the course of the descriptions, the terms vernal (or 
spring) and autwmnad (or general) moult frequently occur; 
which it appears necessary to explain. By the autumnal 
moult is meant that entire annual change of plumage to which 
all birds are liable, and which usually takes place at this time 
of the year, or after the production of the species*. The 
* In the Swallow tribe, and some few others, it happens at a later pe- 
riod, or during the months of January or February. 
