Mixed Singing and Interspecific Territoriality - Consequences of Secondary Contact of Two Ecologically and Morphologically Similar Nightingale Species in Europe

Abstract

Similarities in song, morphology and habitat utilization of Thrush Nightingale and Nightingale, were studied in allopatric and sympatric situations. Responses of the two species to allopatric and sympatric intraspecific and interspecific songs were examined both in allopatry and sympatry. No morphological differences were found between allopatric and sympatric populations. Habitat utilization converged from allopatry to sympatry along most dimensions studied but diverged along one dimension, the topographic gradient: in sympatry the Nightingale used hilly habitats more often than in allopatry; this niche shift of the Nightingale may be connected with the presence of the Thrush Nightingale. In sympatry the two species were interspecifically territorial and playback experiments demonstrated interspecific responses of the two species. The more similar the song was to their own version, the more often the male responded to it. However, sympatric Thrush Nightingales responded similarly to all songs regardless of whether they showed more or less similarity to their own song, and they responded significantly more often to heterospecific songs than did sympatric Nightingales. According to a principal component analysis based on structural characters of the song, there was a one-sided song convergence from allopatry to sympatry among male Thrush Nightingales. However, most sympatric Thrush Nightingale males were mixed-singers with both conspecific and heterospecific songs in their repertoires, and only few males sang only songs similar to those of the Nightingale. In both allopatry and sympatry the Thrush Nightingale males shared more songs, conspecific or interspecific, with their nearest neighbours than with males singing farther away. The possible effects of territory competition between the two species on changes in their distribution are discussed.