ar nesting time, and in winter. For individuals this is obviously the 

 case, but when the entire avifauna of the continent is considered it is 

 found that there are at almost all periods some latitudinal movements. 



Movements of species and groups 



Some species begin their fall migrations early in July and in some 

 parts of the country distinct southward movements can be detected 

 from then until the beginning or middle of winter. For example, 

 many shore birds start south in the early part of July, while the 

 goshawks, snowy owls, redpolls, Bohemian waxwings, and many 

 others do not leave the North until forced to do so by the advent of 

 severe winter weather, or by lack of the customary food. Thus an 

 observer in the northern part of the United States may record an almost 

 unbroken southward procession of birds from midsummer to winter, 

 and note some of the returning migrants as early as the middle of 

 February. While on their way north, purple martins have been known 

 to arrive in Florida late in January and, among late arrivals, the north- 

 ern movement may continue into the first week of June. In some 

 species the migration is so prolonged that the first arrivals in the 

 southern part of the breeding range will have performed their parental 

 duties while others of that species are still on their way north. 



A study of these facts indicates that sometimes there exists a very 

 definite relationship between what we may term northern and southern 

 groups of individuals of the same species. A supposition is that for a 

 species with an extensive latitudinal breeding range, and which has a 

 normal migration, those individuals that nest farthest south migrate 

 first and proceed to the southern part of the winter range; those that 

 occupy the central parts of the breeding range migrate next and travel 

 to regions in the winter range north of those occupied by the first 

 group; and finally the individuals breeding farthest north are the last 

 to start their autumn migration and they remain farthest north during 

 the winter. In other words, this theory supposes that the southward 

 movement of the species is such that the different groups maintain their 

 relative latitudinal position with each other. The black and white 

 warbler furnishes an example. The breeding range of this bird ex- 

 tends west and northwest from northern Georgia and South Carolina 

 to New Brunswick, extending also in a western and northwestern direc- 

 tion as far as Great Bear Lake in northwestern Canada (fig. i). It 



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