ington, 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 {Pas- 

 serella 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. fuligi- 

 7/osa) breeds in the Puget Sound area and makes practically no migra- 

 tion at all, while the other races, nesting on the coast of British Colum- 

 bia 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. 



35 



