shades of marking. In a
similar way there are nine climatic variations of Screech Owls, six
Long-billed Marsh Wrens, and fourteen Horned Larks. It is {121}
difficult to explain why this variation in colour and size is so
pronounced in some species and yet is totally absent in others of
equally wide range. The Mourning Dove breeds in many localities from
the southern tier of Canadian Provinces southward throughout the United
States and Mexico, and yet everywhere over this vast range the birds
are the same in size and colour. Nowhere do the individuals exhibit
any markings suggestive of climatic influences.

Some birds are very rare and are admitted to the list of North American
species because of the fact that during the years a few stragglers from
other parts of the world have been found on our continent. Thus the
Scarlet Ibis from South America, and the Kestrel and Rook from western
Europe, are known to come to our shores only as rare wanderers who had
lost their way, or were blown hither by storms. Eighty-five species of
the birds now listed for North America are of this extra-limital class.
Among those naturally inhabiting the country, some are, of course, much
more abundant than others, thus every one {122} knows that Bald Eagles
are comparatively rare, and that Robins and Chipping Sparrows exist by
millions.

_The Number of Birds in Different States._--The number of kinds of
birds found in any one State depends on the size of the State, its
geographical situation, and the varieties of its climate as affected by
the topography in reference to mountains, coastlines, etc. The number
of bird students and the character of their field studies determine the
extent to which the birds of a State have been catalogued and listed.
The following list indicates the number of kinds of birds that have
been recorded in forty-three of the States and the District of
Columbia. The authority for the statement in each instance and the
year in which the figures were given is also stated:

_Alabama_, 275 (Oberholser, 1909).
_Arizona_, 371 (Cooke, 1914).
_Arkansas_, 255 (Howell, 1911).
_California_, 541 (Grinnell, 1916).
_Colorado_, 403 (Cooke, 1912).
{123}
_Connecticut_, 334 (Sage and Bishop, 1913).
_Delaware_, 229 (Rennock, 1908).
_District of Columbia_, 293 (Cooke, 1913).
_Florida_, 362 (Thurston, 1916).
_Idaho_, 210 (Merrill, 1898).
_Illinois_, 390 (Cory, 1909).
_Indiana_, 321 (Butler, 1898).