Interspecific territorialism in reed warblers: a local effect revealed by playback experiments

Earlier playback experiments on the reed warbler Acrocephalus scirpaceus established a clear case of interspecific territorialism with a sympatric species in West Germany (Catchpole 1978), where male reed warblers responded aggressively to playback of recorded song from a marsh warbler A . palustris . Marsh warblers are locally abundant and regularly breed in adjacent territories where interspecific interactions are common. This is not the case in England and no response was obtained from a population of reed warblers tested with the same experiment . These and other results suggest that interspecific territorialism in Acrocephalus warblers is adaptive and can largely be explained by competition between closely related species with similar ecology . In particular, a species like the reed warbler which arrives back from migration first . and later finds its habitat invaded by a competitor, attempts to exclude the rival species . The relationship between interspecific territorialism and competition has been pointed out by early reviewers (Simmons 1951 ; Orians & Willson 1964) . Later Cody (1969, 1974) suggested a hypothesis of physical or vocal convergence as a special mechanism in the evolution of interspecific territorialism . However, Murray (1971, 1976, 1981) favours the view that interspecific aggression could occur through mistaken identity as misdirected intraspecific aggression . If a general failure to recognize closely related species is responsible for interspecitic aggression, then presumably different populations would show a similar response to playback of closely related species song . By demonstrating a clear differential response related to the presence of a sympatric competitor, the earlier experiment (Catchpole 1978) provided some evidence against the mistaken identity hypothesis . It was concluded that individual males learn to recognize and respond to the songs of congenerics who are regularly encountered in competitive situations. One weakness in the argument is that the experiments were carried out in different countries, and there might he some inherent differences between British and continental populations of reed warblers which could explain the differential response obtained . Reed (1982) has since reported on a differential response between two unrelated competitors . the chaffinch Fringilla coelebs and the great tit Parus mayor, on the mainland and islands off Scotland . In this case there can be no question of confusion in morphology or song structure, and Reed explains his results by the different ecology and competitive relationships on the mainland and islands . We now report another case concerning the reed Short Communications 299