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, 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 situtation 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 blackheaded bunting appears (Fig. 8).
By Age
The adults of most birds leave the young when they are grown. This
gives the parents an opportunity to rest and renew their plumage before
starting for winter quarters. The young are likely to move south together
ahead of their parents. This has been documented in a number of species
including our mourning dove, the common swift of Europe, and storks. Mueller
and Berger (1967) found an age-specific migration pattern in sharp-shinned
hawks passing through Wisconsin. The immatures were much in evidence during
mid-September while the adults came through a month later. Far to the south
in the Antarctic, young Adelie penguins depart for northern wintering grounds
much earlier than adults.
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| 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. |
In a few species, adults depart south before the young. Adult 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 Johnson (1963) did not find this segregation in the Hammond's flycatcher. In Europe, adult red-backed shrikes are known to migrate ahead of their young.
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| 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. |
In contrast to this loss of parental concern, 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 families banded
together. When these flocks separate into small V-shaped units it is probably
correct to assume an old 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. Murray and Jehl (1964) concluded from mist-netting many thousands
of migrant passerines at Island Beach, New Jersey, that adults and juveniles
travel at approximately the same time.
By Sex
Males and females of some species may migrate either simultaneously
or separately. In the latter case it is usually the males, rarely the females,
that arrive first. Sometimes great flocks of male red-winged blackbirds
reach a locality several days before any females; this is particularly
the rule in spring. The first robins are usually found to be males, as
are also the first song sparrows, rose-breasted grosbeaks, 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 territorial
possession whereby the male selects the area where it intends to breed
and each individual attempts to protect a definite territory from trespass
by other males of his own kind, while announcing his presence to rival
males and later arriving females by song or other display. The female then
selects the site where she wishes to nest. The long-billed marsh wren is
a noteworthy example; the males may enthusiastically build several nests
before the females arrive. 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 (Thompson and Person 1963). In the Chicago
area, Annan (1962) reported that some males, such as the hermit thrush,
Swainson's thrush, gray-checked thrush, and veery, arrive before any females
and predominate during the first week of occurrence.
In a few species the males and females apparently arrive at the breeding grounds together and proceed at once to nest building. In fact, among shorebirds, ducks, and geese, courtship and mating often takes place in whole or in part while the birds are in the South or on their way north, so that when they arrive at the northern nesting grounds they are paired and ready to proceed at once with raising their families. Mallards and black ducks may be observed in pairs as early as December, the female leading and the male following when they take flight. Naturally, these mated pairs migrate north in company, and it was largely to protect such pairings that duck shooting in spring was abolished by Federal law.
In the coastal subspecies of the western flycatcher, the sexes appear to migrate in synchrony during the spring in contrast to migration of Hammond's flycatcher in which the adult males usually precede the females (Johnson 1973). Both sexes of the common blackcap of Europe appear to migrate together at least across the eastern end of the Mediterranean during the spring (Fig. 9).
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| 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 preceed the females. |
By Kinds of Flocks
Migratory flights are frequently accomplished in close flock formation,
as with shorebirds, blackbirds, waxwings, and especially some of the buntings,
longspurs, juncos, and tree sparrows. Other species maintain a very loose
flock formation; examples are turkey vultures, hawks, swifts, blue jays,
swallows, warblers, and bluebirds. Still others, the grebes, snowy owls,
winter wrens, shrikes, and belted kingfishers, ordinarily travel alone,
and when several are found in close proximity it is an indication they
have been drawn together by unusual conditions, such as abundant food.
Just as flocking among resident birds provides group protection against predation, flocking in migration greatly facilitates the attainment of destination (Pettingill 1970). The V-shaped flocks often associated with Canada geese have a definite energy conserving function by creating favorable air currents for every member of the flock but the leader; when the leader becomes tired, it will often change places with a member behind. Night migrating flocks generally fly in looser formations than do day migrating flocks.