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, peaks in the migration of the five kinds of spotted thrushes generally do not coincide. Dates of departure in these species have evolved so all the individuals of these closely related birds do not converge on one area at the same time and subsequently exhaust the food supply. By selection of staggered peak migration dates, evolution has distributed the members of this family more or less evenly throughout the entire season. Likewise, in the eastern Mediterranean area, we find a similar situation in spring migration for three closely related buntings; Cretzschmar's bunting comes through first, followed a few weeks later by the Ortolan bunting and, at the tail end of the migration period, the black-headed bunting appears (Fig. 8).