To hybridise or not to hybridise? Systematic review and meta-analysis of woodpeckers

Hybridisation is a common phenomenon among birds in general. Woodpeckers (Picidae) are no exception, as approximately 20% of species are known to hybridise and for many others interspecific mating is suspected. However, the mechanisms and consequences (phenotypic and genetic) of hybridisation are known for only a fraction of woodpecker species. Here, we conduct a systematic review on the literature that deals with hybridisation in woodpeckers and use a meta-analytical approach to examine the available geographical and genetic data. According to available published data, the majority of woodpeckers that hybridise inhabit the Neotropics, followed by the Nearctic and the Palearctic. Hybridisation appears to be less common in the Afrotropic and Oriental regions. As expected, genetic distances are substantially lower between hybridising species pairs than between non hybridising congenerics. This implies that hybridisation is typical for "young" (sister) pairs of species, that typically have similarities in their respective reproductive biology, ecology and ethology, and thus had less time for genetic incompatibilities to build up. These similarities also explain the difficulties experienced in studies on hybrid woodpeckers, but these could be overcome by the use of modern techniques (remote sensing and/or citizen science combining with AI identification and genomic identification). As hybridisation plays a role in woodpecker evolution and ecology it should be considered when conservation policies for rare species are drafted.