WHEN BIRDS MIGRATE 



One ordinarily thinks of the world of birds as sedentary during two 

 periods each year, at nesting time, and in winter. For individuals this 

 is obviously the case, but when the entire avifauna of North America 

 or the world is considered, it is found that at almost all periods there 

 are some latitudinal movements of birds. A few of these movements 

 reoccur year after year with calendar-like regularity. Each species, 

 or group of species, migrates at a particular time of the year and some 

 at a particular time of the day. In this section some of the interesting 

 differences will be discussed as to when birds migrate. 



Time of Year 



Some species begin their fall migrations early in July, and in other 

 species distinct southward movements can be detected late into the 

 winter. While some migrants are still traveling south, some early 

 spring migrants can be observed returning north through the same 

 locality. For example, many shorebirds start south in the early part 

 of July, while the goshawks, snowy owls, redpolls, and Bohemian 

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

 severe winter weather or a lack of 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 

 migrants, the northern movement may continue well into 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 and may actually start south while others of the 

 species are still on their way north. 



A study of these facts indicates the existence of northern and 

 southern populations of the same species that have quite different 

 migration schedules. In fall, migratory populations that nest farthest 

 south migrate first to the winter range because they finish nesting 

 first. For example, the breeding range of the black-and-white 

 warbler covers much of the eastern United States and southern 

 Canada northwest through the prairies to Great Bear Lake in 

 Canada (Fig. 1). It spends the winter in southern Florida, the West 

 Indies, southern and eastern Mexico, Central America, and 

 northwestern South America. In the southern part of its breeding 

 range, it nests in April, but those summering in New Brunswick do 

 not reach their nesting grounds before the middle of May. (Lines that 

 connect points where birds arrive at the same line are called 



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