THE ADAPTIVE SIGNIFICANCE OF ALARM CALLS GIVEN BY SHOREBIRDS ON THEIR WINTER FEEDING GROUNDS.

Abstract

Some wintering shorebirds (Charadrii) fly up and give a loud alarm call when they are disturbed by a bird of prey. The evolution of this behavior is difficult to explain by natural selection as it seems to confer an advantage to nearby birds by warning them of a predator, and may attract the attention of the predator to the caller itself. Maynard Smith (1965) argued that alarm calls probably evolved through kin selection on the breeding grounds, where the risk to the caller would be outweighed by the advantage of the increased survival of its young. However, it is sometimes difficult to explain the persistence of alarm calling through the winter. In many winter flocks, including those formed by shorebirds, the birds are likely to be only distantly related, and 'alarmism' could only be maintained as a result of kin selection if the disadvantage of calling were extremely small. Maynard Smith (loc. cit.) suggests that this would be the case with alarm calls that had acquired properties that made them very difficult to locate. Whilst these characteristics may be present in some passerine calls (Marler, 1955; Perrins, 1968), they do not seem to exist in the alarm calls given by some shorebirds on their winter feeding grounds. For example, oystercatcher Haematopus ostralegus, redshank Tringa totanus, greenshank Tringa nebularia, curlew Numenius arquata and whimbrel Numenius phaeopus fly up and give very loud and easily located calls. One reason for flying up may lie in the tendency of some raptors to take their prey on the ground (e.g., Dickson, 1974; Page and Whitacre, 1975), though other raptor species, such as peregrines Falco peregrinus, often pursue their prey in the air (Hunt et al., 1975). The reason for also giving an alarm call is less clear, and the apparent danger in doing so is exacerbated by the open nature of the habitat and the large size and/or conspicuous plumage of these species. The advantage in shorebirds may be that giving an alarm call enables a flock to form, and this protects the caller from predation. Evidence that grouping can reduce the risk of predation, even after the prey has been detected, is provided by a number of studies. A group may reduce the