How common is hybridization in birds?

Abstract

Hybridization—the interbreeding of different species—plays an integral role in the evolution of numerous bird species. However, it remains unclear how widespread this phenomenon is within and across different bird groups. Estimating the incidence of hybridization in different bird lineages can inform comparative analyses to uncover the evolutionary and ecological forces that promote or prevent the formation of hybrids. Estimates on a species level have varied from about 10% to almost 20% of hybridizing bird species. The variation among the latest estimates largely depends on the choice of global species checklists with differing decisions on the taxonomic status of certain (sub)species. However, the hybrid records in these estimates have generally been taken at face value. Detailed assessments of these records revealed that some cases are unreliable or incorrect. Removing of these faulty records and discovery of novel hybrids will result in a fluctuating percentage of hybridizing species that can best be captured in a living document (e.g., a regularly updated website or checklist). Estimating hybridization on an individual level is more challenging. A first attempt, using citizen science data from the eBird project, reported 0.064% hybrid individuals in the United States. This percentage is probably an underestimate due to underreporting of hybrids by birdwatchers and remains to be finetuned by considering other confounding factors, such as spatial and temporal variation in hybridization dynamics. Future efforts in estimating avian hybridization will need to rely on a combination of data sources and techniques, such as genetics, museum specimens, and citizen science.

Evidence of hybridization between the Blue-winged Teal (Spatula discors) and Red Shoveler (Spatula platalea)Evidencia de hibridización entre pato medialuna Spatula discors y pato cuchara Spatula platalea

“…Hybridization is a process observed in multiple animal species. In birds, it is estimated that between 10 and 20% of species hybridize (Ottenburghs 2023), although the proportion of hybrid individuals is small (0.064%, Justyn et al 2020). Particularly in Anseriformes, hybridization is high and has been observed in almost 60% of species (Ottenburghs et al 2015).…”