INTRODUCTION. xi 
crimination of species and subspecies may be a question of material, a great deal 
depends upon our knowledge of the birds in life, their natural surroundings, and 
other things which can be learned only out of doors. Fortunately, a very large 
number of accomplished field-naturalists have carefully observed the habits of our 
birds, and through their published records have together contributed a vast store 
of information which no single person could himself have gained. To the much 
that has been gleaned from this source have been added the author’s field-notes, col- 
lected during the period extending from a recent date back to the year 1863, and 
embracing many measurements of fresh specimens, notes on location of nests, fresh 
colors of bill, eyes, feet, ete., and various other useful memoranda. 
MEasuREMENTS are in English inches and hundredths, except when otherwise 
stated". Whenever practicable, they have been taken from large series of speci- 
mens, and the extremes given, as well as the average, which follows in parenthesis. 
Thus, 5.75-6.50 (6.12) would indicate the minimum, maximum, and average, re- 
spectively, of the measurements taken. In the case of closely-allied forms, or where 
distinctive characters are largely a matter of dimensions or the proportionate meas- 
urements of different parts, care has been taken to measure, whenever possible, an 
equal number of specimens of the several forms to be compared; and specimens in 
abraded or otherwise imperfect plumage, as well as young birds, have been excluded. 
When there is any marked sexual difference in size, the number of males and females 
measured of allied forms has also been made as nearly equal as possible. Length 
is to be understood as meaning the total length (from end of bill to tip of tail) of 
the fresh specimen, before skinning, unless otherwise stated. This measurement is 
the least important of any for the purpose of determination of the species, being of 
any value at all only when taken from the fresh specimen, before skinning, and even 
then subject to much variation, according to the amount of stretching to which the 
bird is subjected. In a very large number of species the author’s measurements 
from freshly-killed specimens are given, but in perhaps a majority of cases it has 
been necessary to take measurements made by others. Owing to the fact that all 
our leading authorities have been consulted on this point and only the extremes 
given, it has been found impracticable (even if desirable) to distinguish the latter 
from the former, but it may be remarked that nearly all of those in which 
the range of variation between the extremes is very great belong to the latter 
category. Length of wing is from the “bend,” or carpal joint, to tip of longest 
primary, the rule being laid along the outer or convex side and the wing brought 
up close to it for its entire length. Length of tail is from tip of longest feathers to 
their apparent base, a point often very difficult to determine, and subject to more 
or less variation according to the method of preparing the skin, the amount of 
shrinkage to which the flesh at the base of the tail has been subjected, etc. Length 
of culmen is the chord: of the curve, measured with dividers, from the tip of the 
upper mandible to the extreme base of the culmen (this often more or less concealed 
by feathers of the forehead)—unless the qualifying word “exposed” is prefixed, in 
which case the measurement is similarly made to where the frontal feathers close 
1 See foot-note on page xii., as to conversion of inches and decimals into millimetres. 
