20 MICHIGAN BIRD LIFE. 
would have to be considered, for there are at least eighteen other Michigan 
birds, belonging to twelve different families, which have more or less blue 
in-their plumage, and several of them (Kingfisher, Blue Heron, Tufted Tit) 
are also conspicuously crested. Moreover, questions of color are often 
very difficult ones, the average individual being unable to discriminate shades 
nicely, or at least to name them accurately. Color, therefore, has been kept 
out of the artificial keys so far as practicable, and the aim has been to select 
characters for consideration which are clear, definite and readily recognized, 
so that the student can tell at a glance whether the specimen before him 
possesses that character or not. 
Technical terms will be found defined in the glossary near the end of the 
volume, and most of the important structures used in classification are 
illustrated by text figures, a list of which follows the table of contents at the 
beginning of the book. 
MIGRATION. 
The Century Dictionary defines migration as follows: ‘The act of mi- 
grating; change of residence or habitat; removal or transit from one locality 
or latitude to another, especially at a distance.” In further explanation 
the same authority adds, ‘Migration seems to be determined, primarily 
and chiefly, by conditions of food supply, but this does not fully account 
for the apparently needless extent and the wonderful periodicity of the 
movement, nor for the fact that individuals sometimes return to exactly the 
same spot to breed again after passing the winter perhaps thousands of miles 
away.” 
The term migration as applied to birds is familiar to every one, and the fact 
that many of our birds desert us each autumn and return in the spring is so 
familiar that even the most unobservant can scarcely have failed to note it. 
The more careful student will have seen, however, that not all our birds 
leave us in fall, and possibly he may have guessed‘also that those which 
return in the spring are but a fraction of those which withdrew the previous 
year. In all the life-histories in the present work reference will be found 
to the character of residence, and in those species which migrate regularly 
an attempt is made to give approximately the dates of arrival and departure. 
It must be remembered, however, that Michigan covers a long distance from 
north to south (more than 400 miles) and that dates will vary much with 
latitude and other conditions. It seems wise therefore to devote a few pages 
here to a consideration of the facts of bird migration in general as well as in 
our own state. 
Considering merely the condition of residence we may divide our birds 
into four groups: First, residents or permanent residents, those which 
are with us all the year. Second, summer residents, or summer visitors, 
those which nest with us. Third, transients, or birds of passage. Fourth, 
winter visitors or winter residents.* 
Nota few of our common birds are residents in one part of the state and 
only summer visitors or even transients in another, while other species come 
regularly or occasionally into the northern parts of the state in the winter 
but never reach the southern counties. The Snow Bird or Junco and the 
White-throated Sparrow are transients in the southern half of the state, 
but summer residents in the northern half; while the Meadowlark and Mourn- 
* Much of what follows on this subject is taken verbatim from the author’s paper entitled ‘‘Fact 
and Fancy in Bird_Migration.” Eighth Rep. Mich. Acad. Science, 1906 (1907), pp. 13-25. 
