SEGREGATION DURING MIGRATION 



By Individuals or Groups of Species 



During the height of northward movement in spring, the woods 

 and thickets may suddenly be filled with several species of wood 

 warblers, thrushes, sparrows, flycatchers, and other birds. It is 

 natural to conclude they traveled together and arrived simultaneous- 

 ly. Probably they did, but such combined migration is by no means 

 the rule for all species. 



As a group, the wood warblers probably travel more in mixed 

 companies than do any other single family of North American birds. 

 In spring and fall, the flocks are likely to be made up of the adults and 

 young of several species. Sometimes swallows, sparrows, blackbirds, 

 and some of the shorebirds also migrate in mixed flocks. In the fall, 

 great flocks of blackbirds frequently sweep south across the Plains 

 States, with common grackles, red-winged blackbirds, yellow- 

 headed blackbirds, and Brewer's blackbirds included in the same 

 flock. 



On the other hand many species keep strictly to themselves. It 

 would be difficult for any other kind of bird to keep company with the 

 rapid movements of the chimney swift. Besides flight speed, feeding 

 habits or roosting preferences can be so individual as to make 

 traveling with other species incompatible. Nighthawks also fly in 

 separate companies, as do crows, waxwings, crossbills, bobolinks, and 

 kingbirds. Occasionally, a flock of ducks will be observed to contain 

 several species, but generally when they are actually migrating, 

 individuals of each species separate and travel with others of their 

 own kind. 



Although different species generally do not migrate together, we 

 often find many species passing through an area at the same time. If 

 the different kinds of birds observed in a specific area are counted 

 every day throughout the entire migration season, this count often 

 rises and falls much like the bell-shaped curve exhibited when the 

 number of individuals of a given species are counted through the 

 same time period. Figure 7 shows two peaks in the number of species 

 passing through the desert at the north end of the Gulf of Eilat 

 (=Akaba) in the Red Sea. These two peaks happen to coincide with 

 peaks in the numbers of individuals (mostly from the order of 

 perching birds) traveling through the area. Therefore, in the latter 

 part of March and again in April, one notices not only more birds in 

 the area but also more different kinds. 



Closely related species or species that eat the same food organisms 

 are not often found migrating through the same area at the same 

 time. Ornithologists call this species replacement. In North America, 



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