W B

Figure 4.1 Effects of side-winds (W) on the migratory tracks (T) of birds, depending on adjustments of heading (H), leading to full drift (no compensation, 2, 6), partial drift (partial compensation, 4, 8) and no drift (full compensation, 3, 7). Heading and track are the same when there is no side-wind (1, 5). Upper diagrams relate to powered flight, and the equivalent lower diagrams to soaring flight. If a bird is to continue soaring in a thermal, it will inevitably be transported passively over the ground in whichever direction the thermal is being blown (laterally in the diagrams). The bird may compensate for this during the next gliding phase of the flight. Modified from Kerlinger (1989).

The point at which a bird is no longer able to compensate for lateral drift is thus a function of wind speed and direction, as well as the maximum flight speed that the bird itself is able to maintain (called the threshold for drift, Evans 1966b). Several types of drift can thus be distinguished in migratory birds (Figure 4.1). One type occurs when a bird flies along a fixed heading towards its destination, and is drifted from that heading by lateral winds (called full lateral drift). A second type occurs when the bird compensates for lateral wind, but only partly (called partial drift). A third type occurs when a bird attempts no compensation, so flies downwind, getting progressively further off course (called downwind drift). All three types of drift are commonly seen among migrating birds, as is complete compensation.

There are other ways in which birds could minimise the problems created by crosswinds (Alerstam 1979). For example, birds might allow themselves to be drifted partially at high altitudes and then correct for this by overcompensation

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