Mixed Song, Interspecific Competition and Hybridisation in the Reed and Marsh Warblers (Acrocephalus Scirpaceus and Palustris)

Abstract

[1) Three Reed Warblers (Acrocephalus scirpaceus) with a mixed scirpaceus-palustris song, settled into the dry habitat of a Marsh Warbler (A. palustris) colony in the Liege Province of eastern Belgium in 1974 and 1975. One was established in a mixed population of Reed and Marsh Warblers, and the other two, whose songs were extensively tape-recorded, defended territories inside a large population of Marsh Warblers. 2) The mixed songs consisted of a succession of rather short fragments of song of both species, directly juxtaposed. The palustris phrases contrasted with the scirpaceus phrases not only by their highly mimetic nature, but also in their more liquid timbre, more rapid delivery and rhythmic characteristics. In both mixed singers established in the large Marsh Warbler colony, the palustris song occupied longer sequences and was thus predominant over the scirpaceus element. 3) Observation and playback experiments showed that the mixed songs provoked reactions by individuals of both species, and the Reed Warbler mixed singers reacted to the normal song of both species. But Reed and Marsh Warblers with normal songs reacted only to the song of their own species. Various considerations suggest that the mixed singers had probably learnt the song of the Marsh Warbler during their postfledging period. 4) A Reed Warbler mixed singer, established in the colony in 1975, paired with a female Marsh Warbler. The breeding cycle of the mixed pair proceeded normally until both parents abandoned the nestlings at six days old. This was apparently the result of food shortage during a period of very unfavourable weather, which also affected a large proportion of the breeding pairs of Marsh Warbler. 5) Hybridisation is most easily explained by assuming that the female was as attracted by the mixed song as by the normal song of her own species, given that the territorial male Marsh Warblers reacted strongly to it. The close similarities in the breeding behaviour of both species are discussed, as well as the considerable overlap in ecological requirements. Comparative studies of mixed breeding populations are needed to evaluate the degree of inter-specific competition, and to detect more possible cases of mixed song and hybridisation., 1) Three Reed Warblers (Acrocephalus scirpaceus) with a mixed scirpaceus-palustris song, settled into the dry habitat of a Marsh Warbler (A. palustris) colony in the Liege Province of eastern Belgium in 1974 and 1975. One was established in a mixed population of Reed and Marsh Warblers, and the other two, whose songs were extensively tape-recorded, defended territories inside a large population of Marsh Warblers. 2) The mixed songs consisted of a succession of rather short fragments of song of both species, directly juxtaposed. The palustris phrases contrasted with the scirpaceus phrases not only by their highly mimetic nature, but also in their more liquid timbre, more rapid delivery and rhythmic characteristics. In both mixed singers established in the large Marsh Warbler colony, the palustris song occupied longer sequences and was thus predominant over the scirpaceus element. 3) Observation and playback experiments showed that the mixed songs provoked reactions by individuals of both species, and the Reed Warbler mixed singers reacted to the normal song of both species. But Reed and Marsh Warblers with normal songs reacted only to the song of their own species. Various considerations suggest that the mixed singers had probably learnt the song of the Marsh Warbler during their postfledging period. 4) A Reed Warbler mixed singer, established in the colony in 1975, paired with a female Marsh Warbler. The breeding cycle of the mixed pair proceeded normally until both parents abandoned the nestlings at six days old. This was apparently the result of food shortage during a period of very unfavourable weather, which also affected a large proportion of the breeding pairs of Marsh Warbler. 5) Hybridisation is most easily explained by assuming that the female was as attracted by the mixed song as by the normal song of her own species, given that the territorial male Marsh Warblers reacted strongly to it. The close similarities in the breeding behaviour of both species are discussed, as well as the considerable overlap in ecological requirements. Comparative studies of mixed breeding populations are needed to evaluate the degree of inter-specific competition, and to detect more possible cases of mixed song and hybridisation.]