An illustration of what is now known on this subject is found in the case of the robin. This bird occurs in the Middle Atlantic States throughout the year, in Canada only in summer, and along the Gulf coast only as a winter resident. On the Atlantic coast its movements are readily ascertained, since, for example, in the section about Washington, D. C., the breeding robin is the southern variety (_Turdus migratorius archrusterus_) which is found there from the first of April to the last of October, when its place is taken (in smaller numbers) by the northern robin (_Turdus migratorius migratorius_), which arrives about the middle of October and remains until the following April. It is probable that a similar interchange of individual robins occurs throughout a large part of the balance of its range, the hardy birds from the north being the winter tenants in the abandoned summer homes of the southern birds. The red-winged blackbirds that nest in northern Texas are almost sedentary, but in winter they are joined by representatives of other subspecies that nest as far north as the Mackenzie Valley. =Variable migrations within species= 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_) 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 Alaskan 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, 7). Another example of the same kind is the Maryland yellowthroat 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.