This ability to travel with precision over seemingly featureless stretches of land or water is not limited to birds but is likewise possessed by certain mammals, reptiles, fishes, and insects; the well-known migrations of salmon and eels are notable examples.
For an animal to return to a specific spot after a lengthy migration, it must use true navigation to get there. That is, it needs to not only travel in a given compass heading and know where it is at any given time so the course may be altered when necessary but also be able to recognize its goal when it has arrived. It is dangerous to generalize on the means of orientation and navigation in migration; different groups of birds with different modes of existence have evolved different means of finding their way from one place to another (Pettingill 1970). We are only beginning to realize the complexities involved in the many modes of bird orientation and navigation. All we can do in this section is present a brief summary of some of the more important principles involved and the studies that have enhanced our knowledge in the area.
Ability to follow a more or less definite course to a definite goal is evidently part of an inherited faculty. Both the direction and the goal must have been implanted in the bird's genetic code when the particular population became established at its present location. The theory is sometimes advanced that older and more experienced birds lead the way and thereby show the route to their younger companions. This explanation may be acceptable for some species such as geese, swans, and cranes because they stay in family groups, but not for species in which adults and young are known to migrate at different times, especially when young migrate ahead of the adults. As indicated in a previous section on segregation, many North American shorebirds as well as the cuckoos of New Zealand do this. An inherited response to its surroundings, with a definite sense of the goal to be reached and the direction to be followed, must be attributed to these latter birds.
It is well known that birds possess wonderful vision. If they also have retentive memories subsequent trips over the route may well be steered in part by recognizable landmarks. Arguments against the theory of landmark memory are chiefly that unescorted young birds, without previous experience, can find their way to the winter quarters of their species, even if the wintering area has a radically different landscape and vegetation than the breeding grounds. Experimental findings and field observations indicate landmarks are used in navigation by certain birds, but the degree of use varies considerably among the species (Bellrose 1972a).
To a land-dweller traveling the ocean, the vast expanse may seem featureless but the reverse may be true for a seabird blown over land by a storm. In the latter situation the differences in vegetation and topography "obvious" to land-dwellers are completely foreign to a seabird as it has had little previous experience to help interpret these "strange objects." Griffin and Hock (1949) observed the flight behavior of gannets displaced far inland away from their nests. The bird appeared to search randomly until the coastline was met, then the fliers pursued a much more direct course home. Herring gulls, displaced about 250 miles from their nest in 2 consecutive years, returned the second year in one-sixth the time required the first year (Griffin 1943). To birds such as gannets, albatrosses, and shearwaters, which spend almost their entire lives traveling thousands of miles at sea and return to very specific nesting areas, the "featureless ocean expanses" are probably very rich in visual cues. It is difficult to believe a bird dependent on the sea for its livelihood cannot help but be aware of wave direction, islands, reefs, atolls, concentrations of floating flotsam, organisms, currents, clouds over islands, fog belts, etc.
Much migration takes place at night and great stretches of the open sea are crossed to reach destinations. Nights are rarely so dark that all terrestrial objects are totally obscured, and features such as coastlines and rivers are just those that are most likely to be seen in the faintest light, particularly by the acute vision of birds from their aerial points of observation. Even if terrestrial objects are completely obscured on a very dark night, the migrants are still able to assess their surroundings during the day before starting out again.
Some birds, especially colonial seabirds, seem to be able to fly unerringly through the densest fog, particularly in the vicinity of their nest site. Members of the Biological Survey, proceeding by steamer through a dense fog from the island of Unalaska to Bogoslof Island in the Bering Sea, recorded flocks of murres, returning to Bogoslof after quests for food. The birds broke through the wall of fog astern, flew by the vessel, and disappeared into the mists ahead on the same course as the ship. On the other hand, radar observations of migrating birds have indicated strong directional movements on clear nights but often completely random movements in heavily overcast or stormy weather. Possibly some birds can perceive the position of the sun through an overcast as honey bees are known to do. It is less likely the stars could be detected through overcast at night.
Careful studies have been made on the homing instinct in various seabirds such as Laysan albatrosses, Manx shearwaters, and several tropical species of terns. Sooty and noddy terns reach their most northern breeding point on the Dry Tortugas, off the southwest coast of Florida. They are not known to wander any appreciable distance farther north. Displaced breeding birds returned to their nests on the Dry Tortugas after they had been taken on board ship, confined in cages below decks, and carried northward 400 to 800 miles before being released in a region where they had had no previous experience. Likewise, Laysan albatrosses and Manx shearwaters have returned over 3,000 miles in similar homing experiments.
Possibly the "homing instinct," as shown by pigeons, terns, shearwaters, albatrosses, and by the frigatebirds trained as message carriers in the South Pacific, may not be identical with the sense of perceptive orientation that figures in the flights of migratory birds. Nevertheless, it seems closely akin and is probably governed by the same mechanisms. There are good reasons to assume that once we know the processes governing displaced homing we will know, in general, how birds navigate; this question is still far from being answered (Wallraff 1967).
Some students have leaned toward the possible existence of a "magnetic sense" as being the important factor in the power of geographical orientation. The theory was conceived as early as 1855 and reported in 1882 by Viguier. Investigations of this have been conducted by Yeagley (1947) and Gordon (1948) with contradictory results. In 1951, Yeagley incorporated the idea that sensitivity to the effect of the earth's rotary motion through the vertical component of the magnetic field is the means of orientation. The basic question asked of the theory is: "Can birds detect such minute differences in the earth's magnetic field and can these forces affect bird behavior?"
Attempts to demonstrate the effect of radio waves on the navigational ability of birds have produced contradictory results. In some of these tests, homing pigeons released near broadcasting stations have appeared to be hopelessly confused, whereas in others, apparently conducted in the same manner, no effects could be discerned. Before sensitivity of birds to electromagnetic stimuli of any kind can be accepted or rejected, much additional experimental work is necessary.
Human navigators have used the heavenly bodies in determining their course and position for centuries. It would not be surprising then to find other long-distance travelers using the same method. One of the most constant visual cues a migrating bird could use would be the sun's or moon's path and the location of the stars.
Some of the more recent experimental work on bird navigation has been with astronomical (sun) and celestial (star) directional clues. Studies by Kramer, Sauer, and others have indicated a phenomenal inherited ability in birds to use the position of the sun by day and the stars by night to chart their courses. This involves an intricate compensation for daily, seasonal, and geographical changes in the positions of these heavenly bodies. Kramer (1957, 1961) placed diurnal migrants in circular cages and "changed" the position of the sun with mirrors. The birds shifted their position to compensate for these changes. Sauer (1957, 1958), in a fascinating study with nocturnal migrant warblers, placed birds in a round cage open to the sky. These birds oriented in the normal direction for that locality and time of year. He next placed the cage and birds in a planetarium and projected overhead the night sky star patterns for different seasons and localities. The familiar star pattern produced a normal orientation but an unfamiliar sky caused confusion and complete disorientation. These experiments, begun in Germany, are still continuing in other countries with other species. Emlen (1969) used photoperiod manipulation to change the physiological states of spring and fall migratory readiness in indigo buntings. Half the sample of birds were in breeding condition whereas the other half were already past the reproductive stage even though it was spring "outside." When these birds were subjected to a spring star pattern in a planetarium, the birds in spring condition oriented northward but those in autumnal condition oriented southward. Although some results have been negative, by and large the evidence supports the original findings that the sun and stars are visual "landmarks" used by at least some birds as well as bees and probably many other creatures in finding their way home as well as to their winter and summer quarters.
In conclusion, then, we can say this about bird orientation and navigation: 1) many cues are available to birds for migratory guidance and one or several of these may be used by any migrant; 2) different species and groups of birds use different cues, depending on their migration traits; 3) visual cues probably play a predominant role in migration (radar studies have indicated that some birds can maintain their orientation even under completely overcast nights, although they usually become disoriented under such conditions); and 4) long-distance migrants and pelagic species have a much higher developed sense of orientation than those species that migrate only short distances or not at all.