During the height of the northward movement in spring the woods
and thickets may be suddenly filled with several species of wood
warblers, thrushes, sparrows, flycatchers, and others, which
it is natural to conclude have 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 (Compsothlypidae) probably travel more
in mixed companies than do any other single family of North American
birds. The flocks are likely to be made up of several species, spring
and fall, with both adults and young. Sometimes swallows, sparrows,
blackbirds, and some of the shore birds also migrate in mixed flocks.
In fall, great flocks of blackbirds frequently sweep south across
the Plains States, and occasionally one flock will contain bronzed
grackles (_Quiscalus quiscula_), red-winged blackbirds, yellow-headed
blackbirds (_Xanthocephalus xanthocephalus_), and Brewer's blackbirds
(_Euphagus cyanocephalus_).
On the other hand, many species keep strictly to themselves. It would
be difficult for any other kind of bird to keep in company with a
bird of such rapid movements as the chimney swift, which is rarely
found associated with any other species at any season. Nighthawks,
or bullbats, also fly in separate companies, as usually 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 on migration the individuals of each species
separate and travel with others of their own kind. The flocks of
blue geese, previously mentioned in connection with speed of flight
(p. 18), frequently have with them a few of the closely related snow
geese (_Chen hyperborea_), particularly in the eastern part of their
winter range. The proportion here is usually about 10 to 1, but
farther west the numbers of snow geese increase until they outnumber
their blue relatives.
The adults of most perching birds drive the young away when they are
grown, probably to be relieved of the necessity of providing for
them, and also in order that the parents may have opportunity to rest
and renew their plumage before starting for winter quarters. The
young birds are therefore likely to drift together and, having no
further responsibility, may start south ahead of their parents. In
contrast with this indifference on the part of the adults of perching
birds, Canada geese and some others, remain in family groups, the
parent birds undergoing the wing molt that renders them flightless
during the period of growth of their young, so that old and young
acquire their full plumage at the same time and thus are able to
start south together. The large flocks, therefore, are composed
of many families that band together, and when they separate into
V-shaped units it is probably correct to assume that it is an old
bird that leads the group. Where there is segregation of the sexes,
the young birds usually accompany their mothers, as is the case with
some of the ducks. After the females start to incubate their eggs,
the males of most species of ducks flock by themselves and remain
together until fall.
The males and females of some species may migrate either
simultaneously or separately. In the latter case it is usually the
males that arrive first, sometimes great flocks of male birds, as in
the red-winged blackbird, reaching a locality several days before any
of the females. This is particularly the rule in spring; the first
robins are usually found to be males, as also are the first song
sparrows (_Melospiza melodia_), rose-breasted grosbeaks (_Hedymeles
ludovicianus_), and scarlet tanagers (Piranga erythromelas). This
early arrival of the males has been explained on the theory of
territorial possession, under which the male selects the area where
it elects to breed, each individual attempting to protect a definite
territory from trespass by other males of his own kind, at the same
time singing or otherwise announcing his presence and inviting the
later arriving females to examine the territory that he has selected
for nesting. The long-billed marsh wren (_Telmatodytes palustris_)
is a noteworthy example, and the males of this species may
enthusiastically build several dummy nests before the females arrive.
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 the shore birds, ducks, and geese, courtship and mating
may take 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 the
raising of their families. Mallards and black ducks may be observed
in pairs as early as January, 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 species that
duck shooting in spring was abolished by Federal law several, years
ago.
Many shore birds nest well within the Arctic Circle, and it is the
opinion of ornithologists that most of these birds share, at least
in part, the habits of the phalaropes, a family in which the male
assumes the entire care of the eggs and young. If this be true, it
explains why in southern latitudes so many of the earliest fall
arrivals are females that may have deserted the breeding grounds
after the eggs were laid.
Migratory flights are frequently accomplished in close flock
formation, as with the shore birds, blackbirds, and waxwings, and
especially some of the sparrows--the snow buntings (_Plectrophenax
nivalis_), longspurs, juncos, and tree sparrows (_Spizella arborea_).
Other species, however, though they travel in flocks, maintain a very
loose formation; examples are the turkey vultures (_Cathartes aura_),
the hawks, swifts, blue jays, swallows, warblers, and bluebirds.
Still others, the grebes, great horned owls (_Bubo virginianus_),
winter wrens (_Nannus hiemalis_), shrikes, and belted kingfishers
(_Megaceryle alcyon_), for example, ordinarily travel alone, and when
several are found in close proximity it is an indication that they
have been drawn together by unusual conditions, such as abundant food.
=WHERE BIRDS MIGRATE=