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.
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.
WHERE BIRDS MIGRATE
Migration by Populations Within Species
Both length and duration of migratory journeys vary greatly between families, species, or populations within a species. Bobwhite, western quails, cardinals, Carolina wrens, and probably some of the titmice and woodpeckers are apparently almost or entirely nonmigratory. These species may live out their entire existence without going more than 10 miles from the nest where they were hatched.
Many song sparrows, meadowlarks, blue jays, and other species make such short migrations that the movement is difficult to detect because individuals, possibly not the same ones, may be found in one area throughout the year while other individuals that move south may be replaced by individuals from the north. Information on different movements of this type, within a species, can be gained by observing birds marked with numbered bands, colored materials, or identification of racially distinct museum specimens.
The American robin is a good example of this type of movement. This species occurs in the southern United States throughout the year, but in Canada and Alaska only during the summer. Its movements are readily ascertained from study specimens. The breeding robin of the southeastern states is the southern race. In autumn most of its more northern nesters, such as those from Maryland and Virginia move into the southern part of the breeding range or slightly farther south. At about the same time the northern American robin moves south and winters throughout the breeding and wintering range of its smaller and paler southern relative. Thus there is complete overlap of wintering ranges of northern and southern American robin populations, although some individuals of the northern race winter in areas vacated earlier by the southern race.
Among many migratory species there is considerable variation among individuals and populations with respect to distances moved. Certain populations may be quite sedentary while others are strongly migratory, and certain individuals of the same population can be more migratory than others. For example, red-winged blackbirds nesting on the Gulf Coast are practically sedentary, but in winter they are joined by other subspecies that nest as far north as the Mackenzie Valley. In certain populations of the song sparrow and other species, males remain all year on their northern breeding grounds while the females and young migrate south.