Part 5

Some investigators have asserted that the sense of direction has its seat in the ears or nasal passages and thus that the bird is enabled to identify air currents and other phenomena. It has been found that disturbance of the columella, or the semicircular canals of the inner ear, will destroy the homing instinct of the racing pigeon, but experiments in the form of delicate operations, or closing the ears with wax, prove such a serious shock to the sensitive nervous system of the bird that they cannot be considered as affording conclusive evidence. Several years ago careful studies were made of the homing instinct of the sooty and noddy terns (Sterna fuscata and Anoüs stolidus), tropical species that in the Atlantic region reach their most northern breeding point on the Dry Tortugas Islands, off the southwest coast of Florida. They are not known to wander regularly any appreciable distance farther north. It was found that some were able to return to their nests on the Tortugas after they had been taken on board ship, confined in cages below deck, and carried northward distances varying from 400 to 800 miles before being released. Landmarks of all lands were entirely lacking, and the birds certainly were liberated in a region in which they had had no previous experience.

Possibly the "homing instinct" as shown by these terns, by the man-of-war birds (Fregata minor), that are trained and used as message carriers in the Tuamotu, Gilbert, and Marshall Islands, and by the homing pigeon, is not identical with the sense of perceptive orientation that figures in the flights of migratory birds. Nevertheless, it seems closely akin and is probably caused by the same impulses, whatever they may be and however they may be received. It is to be remembered, however, that while homing may involve flight from a point that the bird has never before visited, the flight is always to a known point — that is, the bird's nest— while, on the other hand, the first migratory flight is always from the region of the bird's birth to a region it has never before visited. The spring migration might, of course, be more nearly considered as true "homing."

At the present time some students lean strongly toward the possible existence of a "magnetic sense" as being the important factor in the power of geographical orientation. No direct evidence in support of this has been obtained, but it is not impossible that there may exist some form of physiological sensibility to the phenomena of terrestrial magnetism. The theory as laid down (chiefly by European investigators) is highly complex, but briefly stated it is based on a supposed sensitiveness of birds to the magnetic influences that cause variations in the declination and dip of magnetic needles. Some experimental work already done lends a little support to the theory but it is still far from established.

In concluding this discussion of orientation it is pertinent to point out that the migratory instinct appears to be more or less transitory, that it is not persistent over an extended period. Migratory birds may be arrested en route, either by natural conditions, such as unusual food supplies, or forcibly by the act of man, and detained until the end or nearly the end of the migratory season, and then may not attempt to finish the journey, apparently having lost the migratory impulse. In the fall and early winter of 1929, abundant food and an open season caused an unusual number of mallard ducks to arrest their migration and remain in western Montana and northern Idaho. Later, however, a heavy snowfall with subzero temperatures suddenly cut off the food supply, with the result that great numbers of the birds starved to death, when a flight of a few hours would have carried them to a region of open water and abundant food.

SEGREGATION DURING MIGRATION

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=

DISTANCES OF MIGRATION VARY

Definite evidence shows that both the length and the duration of the migratory journey vary greatly. The bobwhite and the western quails, the cardinal (Richmondena cardinalis), the Carolina wren (Thryothorus ludovicianus), and probably some of the titmice and woodpeckers, which are apparently nonmigratory, may round out their full period of existence without at any time going more than 10 miles from the nest where they were hatched.

SHORT AND UNDETERMINED MIGRATIONS

Song sparrows, meadow larks (Sturnella), blue jays (Cyanocitta cristata), and some other species make such short migrations that the movement is difficult to detect, as individuals may be found in one area throughout the year. Thus, at the southern part of the range there is merely a concentration in winter, the summer individuals being entirely sedentary. Speculation is useless on the distances of individual migration without definite evidence concerning the precise winter quarters of birds that summer in a particular part of the breeding range of the species, but from the records of banded birds important evidence is becoming available. Eventually it may be possible to say definitely just how far the song sparrows that nest in northern New England and the Maritime Provinces of Canada travel to their winter quarters, and whether the blue jays of New York and the upper Mississippi Valley remain throughout the winter in their breeding areas, or move farther south and relinquish their places to individuals from southern Canada.

An illustration of what is now known on this subject is found in the case of the robin. This bird occurs in the Middle Atlantic States throughout the year, in Canada only in summer, and along the Gulf coast only as a winter resident. On the Atlantic coast its movements are readily ascertained, since, for example, in the section about Washington, D. C., the breeding robin is the southern variety (Turdus migratorius achrusterus), which is found there from the first of April to the last of October, when its place is taken (in smaller numbers) by the northern robin (T. m. migratorius), which arrives about the middle of October and remains until the following April. It is probable that a similar interchange of individual robins occurs throughout a large part of the rest of its range, the hardy birds from the north being the winter tenants in the abandoned summer homes of the southern birds.

The red-winged blackbirds that nest in northern Texas are almost sedentary, but in winter they are joined by representatives of other subspecies that nest as far north as the Mackenzie Valley.

VARIABLE MIGRATIONS WITHIN SPECIES

The difference in characters between subspecies has been used by students of migration to discover other interesting facts concerning variations of the migratory flight between closely related birds that breed in different latitudes. The familiar eastern fox sparrow (Passerella iliaca iliaca), for example, breeds from northwestern Alaska to Labrador, and in winter is found concentrated in the southeastern part of the United States. It thus travels a long distance each year. On the west coast of the continent, however, six subspecies of this bird breed in rather sharply delimited ranges, extending from the region of Puget Sound and Vancouver Island to Unimak Island, at the end of the Alaska Peninsula. One of these, known as the sooty fox sparrow (P. i. fuliginosa), breeds in the Puget Sound area and makes practically no migration at all, while the other races, nesting on the coast of British Columbia and Alaska, are found in winter chiefly in California. The races that breed farthest north are in winter found farthest south, illustrating a tendency for those birds that are forced to migrate to pass over those so favorably located that they have no need to leave their breeding areas, while the northern birds settle for the winter in the unoccupied areas farther south (fig. 12).

Another example of the same kind is found in the case of the Maryland yellowthroat (Geothlypis trichas) of the Atlantic coast. Birds occupying the most southern part of the general range are almost nonmigratory, residing throughout the year in Florida, while those breeding as far north as Newfoundland go to the West Indies for the winter, thus passing directly over the home of their southern relatives.

Figure 12.--Migration of Pacific-coast forms of the fox sparrow. The breeding ranges of the different races are encircled by solid lines, while the winter ranges are dotted. The numbers indicate the areas used by the different subspecies, as follows: 1, Shumagin fox sparrow; 2, Kodiak fox sparrow; 3, Valdez fox sparrow; 4, Yakutat fox sparrow; 5, Townsend fox sparrow; 6, sooty fox sparrow. (After Swarth; courtesy of the Museum of Vertebrate Zoology, University of California.)]

The palm warbler (Dendroica, palmarum), which breeds from Nova Scotia and Maine west and northwest to southern Mackenzie, has been separated into two subspecies. Those breeding in the interior of Canada (D. p. palmarum) make a 3,000-mile journey from Great Slave Lake to Cuba, passing through the Gulf States early in October. After the bulk have passed, the palm warblers from the Northeastern States and Provinces (D. p. hypochrysea) drift slowly into the Gulf coast region, where they remain for the winter. Their migratory journey is about half as long as that of the northwestern subspecies.

There is no invariable law governing the distance of migration, although in general it is found that where a species has an extensive range, the subspecies that breed farthest north go farthest south to spend the winter

FALL FLIGHTS NOT FAR SOUTH OF BREEDING RANGES

Some other species that have extensive summer ranges, for instance, the pine warbler (Dendroica pinus), rock wren (Salpinctes obsoletus), field sparrow (Spizella pusilla), loggerhead shrike (Lanius ludovicianus), and black-headed grosbeak (Hedymeles melanocephalus), are found to concentrate during the winter season in the southern part of the breeding range, or to occupy additional territory that is only a short distance farther south. The entire species may thus be confined within a restricted area for the period of winter, and then, with the return of warmer weather, spreads out to reoccupy the full range.