Hybridization between Pied and Collared Flycatchers—sexual selection and speciation theory

Abstract

AbstractThe Pied Flycatcher Ficedula hypoleuca and the Collared Flycatcher F. albicollis hybridize frequently on the islands of Gotland and Öland in the Baltic. Hybrids have highly reduced fertility, but introgression is likely to occur. Mating is incompletely species‐assortative and mixed pairs are common. In Central Europe, Pied Flycatcher males are brown in areas of sympatry, and thus have less resemblance to the black Collared Flycatcher males. It has been suggested that females could use male colour to increase the probability of assortative mating, and hence that a speciation process by reinforcement might be taking place. However, we found no indications that the colour of male Pied Flycatchers had an influence on their risk of disassortative mating, and therefore reinforcement probably cannot explain the geographical pattern of colour variation in Pied Flycatcher males. The song and the white collar of Collared Flycatcher males are features more likely to serve in species recognition. The songs of the two species differ greatly, but in areas of sympatry, “mixed” singing by Pied Flycatcher males was common, and this is likely to increase the risk of hybridization. It seems that interactions between the two species are far more complex than predicted by the theory of speciation by reinforcement.