Some species begin their fall migrations early in July, and in some
parts of the country distinct southward movements can be detected
from then until the beginning or middle of winter. For example, many
shore birds start south in the early part of July, while the goshawks
(_Astur atricapillus_), snowy owls (_Nyctea nyctea_), redpolls
(_Acanthis linaria_), Bohemian waxwings (_Bombycilia garrula_),
and many others do not leave the North until forced to do so by
the advent of severe winter weather or by lack of the customary
food. Thus, an observer in the northern part of the United States
may record an almost unbroken southward procession of birds from
midsummer to winter, and note some of the returning migrants as early
as the middle of February. Purple martins (_Progne subis_) have been
known to arrive in Florida late in January on their way north, and
the northern movement may continue among late arrivals into the first
week of June. In some species the migration is so prolonged that the
first arrivals in the southern part of the breeding range will have
performed their parental duties while others of that species are
still on their way north.
A study of these facts indicates that sometimes there exists a very
definite relationship between what we may term northern and southern
groups of individuals of the same species. A supposition, on which
additional banding work is expected later to give definite facts, is
that in the case of some species that have an extensive latitudinal
breeding range and a normal migration, the individuals that nest
farthest south migrate first in fall and proceed to the southern
part of the winter range; those that occupy the central parts of the
breeding range migrate next, and travel to regions in the winter
range north of those occupied by the first group; and finally the
individuals breeding farthest north are the last to begin their fall
migration and these remain farthest north during the winter. In
other words, this theory supposes that the southward movement of the
species is normally such that the different groups maintain their
relative latitudinal positions, both spring and fall.
[Illustration: B4503M
Figure 2.--Summer and winter homes of the black-and-white warbler. A
very slow migrant, as the birds nesting in the northern part of the
country take 50 days to cross the breeding range. The rapidity of
their advance is shown in figure 3.]
The black-and-white warbler (_Mniotilta varia_) furnishes an example.
The breeding range of this bird extends west and northwest from South
Carolina and New Brunswick as far as Great Bear Lake in northwestern
Canada (fig. 2). The bird spends the winter in southern Florida, the
West Indies, central Mexico, Central America, and northwestern South
America. In the southern part of its breeding range it is nesting in
April, but the summer residents of New Brunswick do not reach their
breeding grounds before the middle of May. Therefore, about 50 days
are required for these northbound birds to cross the breeding range,
and if 60 days be allowed for nest building, egg laying, incubation,
care of the young, and molt, they would not be ready to start
southward before the middle of July (fig. 3). Then another 50-day
trip south, and the earliest migrants from the northern areas would
reach the Gulf coast in September. But both adults and young have
been observed at Key West, Fla., by the middle of July, and on the
northern coast of South America by August 21. Since the birds at Key
West were fully 500 miles south of the breeding-range, it is evident
that they must have come from the southern part of the nesting area.
[Illustration: B4504M
Figure 3.--Isochronal migration lines of the
black-and-white warbler, showing a slow and uniform migration, the
advance across the United States being apparently only about 20 miles
a day.]
Many similar cases might be mentioned, such as the black-throated
blue warblers (_Dendroica caerulescens_), which are still observed
in the mountains of Haiti in the middle of May when others of the
species are en route through North Carolina to breeding territory in
New England or have even reached that region. Redstarts (_Setophaga
ruticilla_) and yellow warblers (_D. aestiva_), evidently the more
southern breeders in each case, are seen returning southward on the
northern coast of South America just about the time that the earliest
of those breeding in the North reach Florida on their way to winter
quarters.
[Illustartion: B2282M
Figure 4.--The Kentucky warbler, a night migrant, in
traveling to its winter quarters in Central America and northwestern
South America, uses route no. 5, figure 20.]