Ongoing climate change is expected to result in rearranged species distributions. Closely related species that have been separated for millions of years will come into contact and may drive each other to extinction through ecological interactions (e.g. competition) or through hybridization.

Simulation models suggest that hybridization increases the risk of extinction beyond the risk resulting from interspecific competition alone. However, the combined effects of hybridization and interspecific competition are seldom considered in cases of species replacements. Here we investigate the joint effects of competition and hybridization in two closely related species of birds, collared (Ficedula albicollis) and pied flycatchers (F. hypoleuca).

Collared and pied flycatchers co-occur in central and eastern Europe and on the Swedish Baltic isles of Öland and Gotland. We have previously shown that a life-history trade-off between competitive ability and robustness to harsh environment promotes regional coexistence of the two species. On Öland, pied flycatchers are declining in the area where collared flycatchers first colonized the island in the 1960’s. In this study we show that pied flycatchers are approaching local extinction within this area, and we investigate the underlying reasons for this rapid exclusion.

Results/Conclusions

Neither reproductive lifespan nor breeding success differed significantly between the two species. Local recruitment was relatively lower for pied flycatchers, but it was in particular increased competition over nest-sites that hindered mainly young immigrant male pied flycatchers in establishing a territory. In addition, the exclusion of female pied flycatchers lagged behind the exclusion of males. As the relative frequency of males became biased towards collared flycatchers, the risk of hybridization increased for female pied flycatchers, speeding up the exclusion process. Hence, the relative roles of competition over nest sites and hybridization varied during the process of exclusion, and also reinforced each other.

Our results demonstrate that the combined effects of various interactions between species may be more relevant to study than determining their relative importance during the process of species replacement. These findings are particularly interesting in the light of recent climate change since the expected range shifts of many taxa will increase the risk of competitive and sexual interactions between previously separated species.