Collared Flycatcher

In the 1960s, collared flycatchers expanded their breeding range northward, colonizing the Swedish island of Öland in the Baltic Sea. This expansion, a response to climate change, brought them into sympatry with the European pied flycatcher. The breeding population dynamics and hybridization between these two species have been monitored on the island since 2002, giving scientists the rare opportunity to study life-history traits, hybridization, climate adaptation, and speciation. On Öland, the socially dominant collared flycatcher, which rely heavily on high-quality habitat, have displaced most European pied flycatcher to poorer breeding habitats. Although hybridization occurs in approximately 4% of the population, there is a strong selection against hybrids. The resulting F1 hybrids of both sexes have lower fitness and are sterile: females experience hatching failure, while males suffer from impaired sperm morphology.