The difference in characters between subspecies has been used by
students of migration to discover other interesting facts concerning
variations of the migratory flight between closely related birds
that breed in different latitudes. The familiar eastern fox sparrow
(_Passerella iliaca iliaca_), for example, breeds from northwestern
Alaska to Labrador, and in winter is found concentrated in the
southeastern part of the United States. It thus travels a long
distance each year. On the west coast of the continent, however, six
subspecies of this bird breed in rather sharply delimited ranges,
extending from the region of Puget Sound and Vancouver Island to
Unimak Island, at the end of the Alaska Peninsula. One of these,
known as the sooty fox sparrow (_P. i. fuliginosa_), breeds in the
Puget Sound area and makes practically no migration at all, while the
other races, nesting on the coast of British Columbia and Alaska,
are found in winter chiefly in California. The races that breed
farthest north are in winter found farthest south, illustrating a
tendency for those birds that are forced to migrate to pass over
those so favorably located that they have no need to leave their
breeding areas, while the northern birds settle for the winter in the
unoccupied areas farther south (fig. 12).
Another example of the same kind is found in the case of the Maryland
yellowthroat (_Geothlypis trichas_) of the Atlantic coast. Birds
occupying the most southern part of the general range are almost
nonmigratory, residing throughout the year in Florida, while those
breeding as far north as Newfoundland go to the West Indies for
the winter, thus passing directly over the home of their southern
relatives.
[Illustration: B4774AM
Figure 12.--Migration of Pacific-coast forms of the fox sparrow.
The breeding ranges of the different races are encircled by solid
lines, while the winter ranges are dotted. The numbers indicate the
areas used by the different subspecies, as follows: 1, Shumagin fox
sparrow; 2, Kodiak fox sparrow; 3, Valdez fox sparrow; 4, Yakutat
fox sparrow; 5, Townsend fox sparrow; 6, sooty fox sparrow. (After
Swarth; courtesy of the Museum of Vertebrate Zoology, University of
California.)]
The palm warbler (_Dendroica, palmarum_), which breeds from Nova
Scotia and Maine west and northwest to southern Mackenzie, has been
separated into two subspecies. Those breeding in the interior of
Canada (_D. p. palmarum_) make a 3,000-mile journey from Great Slave
Lake to Cuba, passing through the Gulf States early in October. After
the bulk have passed, the palm warblers from the Northeastern States
and Provinces (_D. p. hypochrysea_) drift slowly into the Gulf coast
region, where they remain for the winter. Their migratory journey is
about half as long as that of the northwestern subspecies.
There is no invariable law governing the distance of migration,
although in general it is found that where a species has an extensive
range, the subspecies that breed farthest north go farthest south to
spend the winter