Birds have at least two orientation systems: inherited clock-and-compass orientation enables naive young birds to find appropriate wintering areas, and bi-coordinate navigation enables experienced birds to home to a known area. Both groups use celestial (sun, skylight polarisation and stars) or magnetic information (inclination and intensity of force lines) as cues to compass direction. The sun compass must be time-compensated to allow for time of day, so an important component of this mechanism is the bird's internal time sense. Birds also use known landmarks to recognise previous travel routes and home areas, and perhaps also to help maintain a straight course. Use of all known compasses employed by migratory birds can be modified by experience.
At the very least, inexperienced juvenile birds have an innate ability to fly on a straight compass course for an appropriate time to reach their wintering areas; they can change directional preference between autumn and spring, and even at different stages in the same migratory journey. This is apparently achieved by having an inherent seasonal directional preference and an internal calendar clock (endogenous rhythm) to switch migratory behaviour on and off at appropriate times. Birds also know the positions of areas previously visited, and can return to their natal areas, or to their previous breeding or wintering sites at a later date, or if experimentally displaced. In addition to a simple 'clock-and-compass' system, therefore, birds also have a map sense which enables them to find familiar places again, often by direct flight. Exactly how they achieve this remains a mystery, despite our knowledge of the directional cues involved.
Many landbirds that cross deserts or oceans accumulate the necessary fuel reserves just before embarking on the crossing, which may occur part way through their journey. Others change direction at particular points in their journeys. They evidently 'know' that they have reached an appropriate place to accumulate large body reserves or change their heading. One possible mechanism involves the inherent time responses: after a given period after the start of migration, the bird changes its behaviour. Another involves response to the specific conditions found at particular sites, notably magnetic conditions.
Birds can also correct for drift by crosswinds, a facility which is often more apparent in adults than in juveniles, but is clearly present in both. In-flight compensation for drift could occur through reference to the ground below, but in strong winds birds are sometimes blown far off course, yet they are later able to re-orient and get back on course. This applies to juveniles making their first migration as well as to experienced adults, and implies the existence of some course recording and correction mechanism.
In some bird species, young migrate singly, and later in the year than adults, so could not learn specific migration routes from more experienced individuals. In other species, especially cranes, geese and swans, the young are influenced by the behaviour of their parents from whom they learn migration routes and stopping places. In such species, experiments have shown that social influences can sometimes override inherent migratory and directional tendencies.
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