Migratory behavior

Migratory behavior

Regularly occurring, oriented seasonal movements of individuals of many animal species. The term migration is used to refer to a diversity of animal movements, ranging from short-distance dispersal and one-way migration to round-trip migrations occurring on time scales from hours (the vertical movements of aquatic plankton) to years (the return of salmon to their natal streams following several years and thousands of kilometers of travel in the open sea).

Many temperate zone species, including many migrants, are known to respond physiologically to changes in the day length with season (photoperiodism). For example, many north temperate organisms are triggered to come into breeding condition by the interaction between the lengthening days in spring and their biological clocks (circadian rhythms). Similar processes, acting through the endocrine system, bring animals into migratory condition.

To perform regular oriented migrations, animals need some mechanism for determining and maintaining compass bearings. Animals use many environmental cues as sources of directional information. Work with birds has shown that species use several compasses.

Many species of vertebrates and invertebrates possess a time-compensated Sun compass. With such a system, the animal can determine absolute compass directions at any time of day; that is, its internal biological clock automatically compensates for the changing position of the Sun as the Earth rotates during the day. Many arthropods, fish, salamanders, and pigeons can perceive the plane of polarization of sunlight, and may use that information to help localize the Sun even on partly cloudy days.

Only birds that migrate at night have been shown to have a star compass. Unlike the Sun compass, it appears not to be linked to the internal clock. Rather, directions are determined by reference to star patterns which seem to be learned early in life.

Evidence indicates that several insects, fish, a salamander, certain bacteria, and birds may derive directional information from the weak magnetic field of the Earth. See Magnetic reception (biology)

Many kinds of animals show the ability to return to specific sites following a displacement. The phenomenon can usually be explained by familiarity with landmarks near “home” or sensory contact with the goal. For example, salmon are well known for their ability to return to their natal streams after spending several years at sea. Little is known about their orientation at sea, but they recognize the home stream by chemical (olfactory) cues in the water. The young salmon apparently imprint on the odor of the stream in which they were hatched. Current evidence indicates that birds imprint on or learn some feature of their birthplace, a prerequisite for them to be able to return to that area following migration. On its first migration, a young bird appears to fly in a given direction for a programmed distance. Upon settling in a wintering area, it will also imprint on that locale and will thereafter show a strong tendency to return to specific sites at both ends of the migratory route.

Only in birds can an unequivocal case be made for the existence of true navigation, that is, the ability to return to a goal from an unfamiliar locality in the absence of direct sensory contact with the goal. This process requires both a compass and the analog of a map. Present evidence suggests that the map is not based on information from the Sun, stars, landmarks, or magnetic field. Other possibilities such as olfactory, acoustic, or gravitational cues are being investigated, but the nature of the navigational component of bird homing remains the most intriguing mystery in this field.