26 CIRCULAR 363, U. S. DEPARTMENT OF AGRICULTURE 
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 N orth 
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 (Huphagus 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, wax- 
wings, 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), fre 
quently 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 eges, 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 simul- 
taneously 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 (Me- 
lospiza melodia), rose-breasted grosbeaks (Hedymeles ludovicianus), and 
scarlet tanagers (Piranga erythromelas). ‘This early arrival of the 
