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Migration of Birds

Segregation During Migration

As Individuals or Groups of Species

During the height of northward movement in spring, the woods and thickets may suddenly be filled in the morning with several species of wood warblers, thrushes, sparrows, flycatchers, and other birds. It is natural to conclude they traveled together and arrived simultaneously. 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 Great Plains 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. Common Nighthawks fly in separate companies, as do American Crows, Cedar Waxwings, Red Crossbills, Bobolinks, and Eastern Kingbirds. And 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 species-specific as to make traveling with other species incompatible. 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.

Even if different species 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 coincide with peaks in the numbers of individuals (mostly perching birds) traveling through the area. Therefore, in the latter part of March and again in April, there are not only more birds in the area, but also more species.

Figure 7: Graph showing average number of species captured daily during  March, April, and May
Figure 7.  Average number of species captured daily in mist nets during spring migration at Eilat, Israel, in 1968. The number of species passing through an area on migration will rise and fall similar to the number of birds counted in the area. In this case two major movements came through about 1 month apart.

Closely related species or species that eat the same food are not often found migrating through the same area at the same time. In North America, peaks in the migration of the five species 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, the processes involved in evolution have 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 during 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 end of the migration period, the Black-headed Bunting appears (Figure 8). Many groups of migrating species like shorebirds, blackbirds, waxwings, and buntings maintain a close flock formation. Other species like Turkey Vultures, hawks, swifts, Blue Jays, swallows, and warblers maintain a loose flock. And still others, like shrikes, Belted Kingfishers, grebes, and Winter Wrens, ordinarily travel alone.

Figure 8: Graph showing average number Cretzschmar's, Ortolan, and Black-headed Buntings captured daily during  March, April, and May
Figure 8.  Average number of three species of buntings captured daily in mist nets during spring migration at Eilat, Israel, in 1968. Closely related species that migrate through the same area often appear at different times. Thus species that may eat the same foods do not compete with each other.

Just as flocking among resident birds provides group protection against predators and facilitates food finding, flocking of migrants probably serves the same purposes. The V-shaped flocks associated with Canada Geese and Double-crested Cormorants have a definite energy conserving function by allowing members of the flock to gain an aerodynamic advantage from the wing-tip vortices of the bird ahead. It has also been observed from radar studies that day migrants fly in tighter formations than flocks migrating by night. This again may reflect a strategy to deter the effect of aerial predators.

By Age

The adults of most birds abandon the young when they are grown. This gives the parents an opportunity to renew their plumage and gain fat stores before starting for winter quarters. The young may move south together ahead of their parents, as has been documented in a number of species including Mourning Doves and the Common Swift and White Storks in Europe. In Sharp-shinned Hawks passing through Wisconsin, the immatures are much in evidence during mid-September while the adults come through a month later. Far to the south in Antarctica, young Adelie Penguins depart for coastal wintering grounds much earlier than adults.

In a few species, adults depart south before the young. Adult American Golden-Plovers, Hudsonian Godwits, and probably most of the arctic breeding shorebirds leave the young as soon as they are capable of caring for themselves and set out for South America ahead of the juveniles. Likewise, data for the Least Flycatcher indicate adults migrate before the young, but this segregation does not occur in the closely related Hammond's Flycatcher. In Europe, adult Red-backed Shrikes are known to migrate ahead of their young. In contrast, geese, swans, and cranes remain in family groups throughout migration. The parent birds undergo a wing molt that renders them flightless during the period of growth of their young so that both the adults and immatures acquire their flight capabilities at the same time and are able to start south together. Large flocks of Canada Geese, for example, are composed of many family groups. When these flocks separate into small V-shaped units it is probably correct to assume an older goose or gander is leading the family. After female ducks start to incubate their eggs, the males of most species of ducks flock by themselves and remain together until fall. When segregation of the sexes such as this occurs, the young birds often accompany their mothers south.

By Sex

Males and females may migrate either simultaneously or separately. Although there are exceptions, generally passerine males arrive before females. Thus, in spring great flocks of male Red-winged Blackbirds reach a locality several weeks before any females. The first American Robins are usually found to be males, as are the first Song Sparrows, Rose-breasted Grosbeaks, Dickcissels, and Scarlet Tanagers. In Europe, the three buntings mentioned previously are also segregated as to sex during migration. Figure 8 shows two prominent peaks for both the Cretzschmar's and Ortolan buntings; during passage the first peak was primarily males while the second peak consisted mostly of females. This early arrival of males on the breeding grounds is associated with the establishment of territories in which each male defends a definite area from trespass by other males of his own kind, while announcing his presence to rival males and later arriving females by song or other displays. The female then selects the site where she wishes to nest. In the fall, Common and King eiders are sexually segregated during migration. During July, flocks crossing Point Barrow are composed almost entirely of males, while after the middle of August the flocks are almost all females. In the Chicago area, male Hermit Thrushes, Swainson's Thrushes, Gray-cheeked Thrushes, and Veerys arrive before any females and predominate during the first week of passage.

In a few species the males and females arrive at the breeding grounds together and proceed at once to nest. In fact, among shorebirds, ducks, geese, and the Osprey courtship and mating often takes place while the birds are in the South or on their way north, so that when they arrive on the northern nesting grounds, they are paired and ready to proceed at once with raising their families. Mallards and American Black Ducks may be observed in pairs as early as December, the female leading and the male following when they take flight.

In the Pacific-slope Flycatcher, the sexes appear to migrate in synchrony during the spring in contrast to migration of the closely related Hammond's Flycatcher in which the adult males usually precede the females. Both sexes of the Common Blackcap of Europe appear to migrate together at least across the eastern end of the Mediterranean during the spring (Figure 9).

Figure 9: Line graph showing numbers of male and female Blackcaps captured during March, April, and May
Figure 9.  Numbers of male and female blackcaps captured daily in mist nets during spring migration at Eilat, Israel, in 1968. At this point in their migration the sexes are passing through the area at the same time. In other species (e.g., the buntings in Fig. 8), the males often precede the females.

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