Spatial overlap and phenotypic similarity predict hybridisation in birds
Hybridisation is widespread in birds and has important eco-evolutionary implications. Historically, much research has focused on the consequences of hybridisation. To date, however, far less attention has been paid to the causes of hybridisation and, in particular, why particular pairs of species form hybrids. Using large-scale comparative analyses, we find that similarity in plumage, vocalisations, and morphometric traits, as well as evolutionary relatedness, drive hybridisation in North American and European birds. In passerines, the avian order including songbirds, similarity in vocalisations and morphology are the key phenotypic predictors of hybridisation, whereas in non-passerines, species pairs with similar plumage and morphology are more likely to hybridise. We also find that the likelihood of hybrid formation increases with the magnitude of range and habitat overlap, demonstrating that hybridisation is not just a product of occasional contact between closely related species. Our findings have important conservation implications in this age of rapid anthropogenic change and species reassembly.