Читать книгу Migration of Birds онлайн

19 страница из 25

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 extends west and northwest from northern Georgia and South Carolina to New Brunswick, extending also in a western and northwestern direction as far as Great Bear Lake in northwestern Canada (fig. 1). It spends the winter in southern Florida, the West Indies, central Mexico, Central America, and northwestern South America. In the southern part of its breeding range it is nesting in April, but those that summer in New Brunswick do not reach their nesting grounds before the middle of May. Therefore, about 50 days are required for these northbound birds to cross the breeding range, and if 60 days be allowed for nest building, egg laying, incubation, care of young, and molt, they would not be ready to start southward before the middle of July (fig. 2.). Then another 50-day trip south, and the earliest migrants from the northern areas would reach the Gulf Coast in September. But both adults and young have been observed at Key West, Fla., by the middle of July, and on the northern coast of South America by August 21. Since the birds at Key West were fully 500 miles south of the breeding range, it is evident that they must have come from the southern part of the nesting area.