population exhibit "leap-frog" migration patterns. The familiar 

 eastern fox sparrow breeds from northeastern Manitoba to 

 Labrador, but during the winter it is found concentrated in the 

 southeastern part of the United States. On the west coast of the 

 continent, however, a study of museum specimens by Swarth (1920), 

 indicated six subspecies of this bird breeding in rather sharply 

 delimited ranges extending from Puget Sound and Vancouver Island 

 to Unimak Island, at the end of the Alaskan Peninsula. One of these 

 subspecies, known as the sooty fox sparrow, breeds from the Puget 

 Sound-Vancouver Island area northward along part of the coast of 

 British Columbia. It hardly migrates at all, while the other races, 

 nesting on the coast of Alaska, are found in winter far to the south in 

 Oregon and California. Although much overlap exists, the races 

 breeding farthest north generally tend to winter farthest south. This 

 illustrates a tendency for those populations forced to migrate to pass 

 over those subspecies so favorably located as to be almost sedentary. 

 If the northern birds settled for the winter along with the sedentary 

 population, winter requirements may not be as sufficient as in the 

 unoccupied areas farther south (Fig. 10). Therefore, natural selection 

 has insured the different populations will survive the winter by 

 separating the subspecies into different wintering areas. 



Another example of this "leap-frog" migration is illustrated by the 

 common yellowthroat of the Atlantic coast. Birds occupying the most 

 southern part of the general range are almost nonmigratory and 

 reside throughout the year in Florida, whereas the population that 

 breeds as far north as Newfoundland goes to the West Indies for the 

 winter. Thus the northern population literally "jumps" over the home 

 of the southern relatives during migratory journeys. 



The palm warbler breeds from Nova Scotia and Maine west and 

 northwest to southern Mackenzie. The species has been separated 

 into two subspecies: those breeding in the interior of Canada and 

 those breeding in northeastern United States and Canada. The 

 northwestern subspecies makes a 3,000-mile journey from Great 

 Slave Lake to Cuba and passes through the Gulf States early in 

 October. After the bulk of these birds have passed, the eastern 

 subspecies, whose migratory journey is about half as long, drifts 

 slowly into the Gulf Coast region and remains for the winter. 



Fall Flights Not Far South of Breeding Range 



Some species have extensive summer ranges (e.g., the pine 

 warbler, rock wren, field sparrow, loggerhead shrike, and 

 blackheaded grosbeak) and concentrate during the winter season in 

 the southern part of the breeding range or occupy additional 

 territory only a short distance farther south. The entire species may 

 thus be confined within a restricted area during winter, but with the 

 return of warmer weather, the species spreads out to reoccupy the 

 much larger summer range. 



Many species, including the tree sparrow, snow bunting, and 

 Lapland longspur, nest in the far north and winter in the eastern 



42 



