Data from: Reproductive character displacement of female, but not male song discrimination in an avian hybrid zone

Divergence of male sexual signals and female preferences for those signals often maintains reproductive boundaries between closely related, co-occurring species. However, contrasting sources of selection, such as interspecific competition, can lead to weak divergence or even convergence of sexual signals in sympatry. When signals converge, assortative mating can be maintained if the mating preferences of females diverge in sympatry (reproductive character displacement; RCD), but there are few explicit examples. Pied flycatchers (Ficedula hypoleuca) are sympatric with collared flycatchers (F. albicollis) on the Baltic island of Öland, where males from both species compete over nestboxes, their songs converge, and the two species occasionally hybridize. We compare song discrimination of male and female pied flycatchers on Öland and in an allopatric population on the Swedish mainland. Using field choice trials, we show that male pied flycatchers respond similarly to the songs of both species in sympatry and in allopatry, while female pied flycatchers express stronger discrimination against heterospecific songs in sympatry than in allopatry. These results are consistent with RCD of song discrimination of female pied flycatchers where they co-occur with collared flycatchers, which should maintain species assortative mating despite convergence of male sexual signals.

雄性性信号与雌性择偶偏好的分化，通常可维持近缘同域分布（sympatry）物种间的生殖隔离边界。然而，种间竞争等相悖的选择压力，可导致同域分布类群的性信号出现分化减弱甚至趋同。当信号发生趋同时，若雌性择偶偏好于同域环境下发生分化（生殖特征置换（reproductive character displacement, RCD）），仍可维持选型交配（assortative mating），但此类明确实证案例较为匮乏。斑姬鹟（Pied flycatcher, Ficedula hypoleuca）与领姬鹟（collared flycatcher, F. albicollis）在波罗的海厄兰岛（Öland）上同域分布，二者雄性会争夺人工巢箱，鸣曲趋于一致，且两物种偶尔会发生杂交。我们对比了厄兰岛同域种群与瑞典大陆异域种群（allopatric population）中斑姬鹟雌雄个体对鸣曲的识别能力。通过野外选择实验（field choice trials），我们发现：无论在同域还是异域环境中，雄性斑姬鹟对两种鸣曲的响应模式均相似；而雌性斑姬鹟在同域环境下对异种鸣曲（heterospecific songs）的识别强度显著高于异域种群。上述结果与斑姬鹟雌性鸣曲识别能力的生殖特征置换现象相符——当它们与领姬鹟同域共存时，即便雄性性信号发生趋同，该置换仍可维持物种间的选型交配。