Wilkins MR, Scordato ESC, Semenov GA, et al. Global song divergence in barn swallows (Hirundo rustica): exploring the roles of genetic, geographical and climatic distance in sympatry and allopatry. Biological Journal of the Linnean Society. 2018. Available at: https://academic.oup.com/biolinnean/advance-article/doi/10.1093/biolinnean/bly012/4943524.