INTRODUCTION. 15 
Scattered through the following pages will be found numerous so-called 
keys for the separation of suborders, families, and species, and a little exami- 
nation of these will show that the plan upon which they are constructed is 
based upon the presence or absence of certain characters or sets of characters, 
so that the student with specimen in hand simply needs to determine first 
which one of two statements fits the specimen in hand, his decision in this 
case leading to the comparison of two other statements, and so, step by step, 
he is led to recognize in his specimen the existence of certain definite peculiari- 
ties and eventually reaches a conclusion applicable only to a single order, 
family or species, as the case may be, and that the one represented by the 
bird in hand. Having thus located the order, suborder or family to which 
his specimen belongs he turns to the section describing the members of that 
group and there finds another key which in the same way will lead to a 
smaller group and this in turn to the species. Probably the beginner will 
be struck at once by the fact that some characters which he has supposed 
quite important are rarely selected, while others which may seem trivial 
are very largely used. Thus, for example, color or color pattern is used much 
less frequently than size or the proportions of various parts. This naturally 
leads to a discussion of the facts which make it possible to arrange these 
artificial keys. 
At the outset it should be made clear that all the individual birds belong- 
ing to the same species are essentially alike in size, proportions and color, 
after suitable allowance has been made for sex, age and season. A bird is 
said to be adult when it is old enough to breed, and in most cases this point 
is reached after a single year’s growth. But such a breeding bird is not 
necessarily mature in the scientific sense, as shown by the fact that its plumage 
may change repeatedly and for several years before its color pattern and the 
colors themselves become permanent. This is seen clearly in the Bald Eagle, 
which does not get the pure white head and tail until at least three years of 
age, sometimes doubtless not before the fourth year, though yearling birds 
are believed to nest as arule. Essentially the same thing is true of many 
other birds of prey, as well as of many water birds. These cases nevertheless 
are exceptional and most of our smaller birds become practically mature 
in a single year, though it seems certain that they may improve somewhat 
in appearance for several successive years. This is noticeable in such birds 
as tanagers, grosbeaks, bluebirds, and others in which the males are brightly 
colored. There is much variation in the perfection and brilliancy of plumage 
among these males, some retaining considerable areas of the dress charac- 
teristic of immaturity, while others, apparently the most robust and vigorous, 
exhibit the characteristic plumage in the highest degree of perfection. Possi- 
bly, or even probably, these variations are not entirely due to difference in 
age, yet it seems fairly certain that this is the most universal factor. As 
regards size, however, there is little or no change after the first year (except 
in a very few cases), and as a rule young birds old enough to fly and care for 
themselves are as large as their parents. For example, young robins or 
bluejays two weeks after leaving the nest have practically the same measure- 
ments (however, not the same weight) as their parents. Every observant 
person, however, knows perfectly well that the young robin of this age is still 
heavily spotted on breast and back, in this respect entirely unlike its parents, 
while the young bluejay of the same age resembles its parents much more 
closely, the differences lying not so much in color or pattern, as in the lax 
and fluffy texture of the plumage, the decidedly shorter crest, and a certain 
dullness and lack of brilliance about the colors themselves. 
