Juan Carlos Illera1, Juan Carlos Rando2, Martim Melo3,4,5,6, Luís Valente7,8 and Martin Stervander9,10 1Biodiversity Research Institute (CSIC-Oviedo University-Principality of Asturias), University of Oviedo, Mieres 33600, Asturias, Spain 2Departamento de Biología Animal, Edafología y Geología, Universidad de La Laguna, La Laguna 38206, Tenerife, Spain 3CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, InBIO Laboratório Associado, Universidade do Porto, Vairão 4485-661, Portugal 4BIOPOLIS Program in Genomics, Biodiversity and Land Planning, CIBIO, Vairão 4485-661, Portugal 5Museu de História Natural e da Ciência da Universidade do Porto, Porto 4050-368, Portugal 6FitzPatrick Institute of African Ornithology, University of Cape Town, Cape Town 7701, South Africa 7Naturalis Biodiversity Center, 2333 CR Leiden, The Netherlands 8Groningen Institute for Evolutionary Life Sciences, University of Groningen, Groningen 9700 AB, The Netherlands 9Bird Group, Natural History Museum, Tring HP23 6AP, Hertfordshire, United Kingdom 10Department of Natural Sciences, National Museums Scotland, Edinburgh EH1 1JF, United Kingdom Correspondence: illerajuanuniovi.es; jcillera3hotmail.com; martinstervander.com

Understanding the mechanisms underlying species formation and differentiation is a central goal of evolutionary biology and a formidable challenge. This understanding can provide valuable insights into the origins of the astonishing diversity of organisms living on our planet. Avian evolutionary radiations on islands have long fascinated biologists as they provide the ideal variation to study the ecological and evolutionary forces operating on the continuum between incipient lineages to complete speciation. In this review, we summarize the key insights gained from decades of research on adaptive and nonadaptive radiations of both extant and extinct insular bird species. We present a new comprehensive global list of potential avian radiations on oceanic islands, based on published island species checklists, taxonomic studies, and phylogenetic analyses. We demonstrate that our understanding of evolutionary processes is being greatly enhanced through the use of genomic tools. However, to advance the field, it is critical to complement this information with a solid understanding of the ecological and behavioral traits of both extinct and extant avian island species.