On the rarity of ratite hybridization

Although an estimated 10–20% of bird species are known to hybridise, research on avian hybridisation is strongly biased towards certain groups, leaving others understudied. Hybridisation in ratites–a polyphyletic group of flightless birds including ostriches, rheas, cassowaries, emus, and kiwi–has received little attention, largely due to its apparent rarity. In this study, I review the available evidence for hybridisation in ratites and identify four well-supported cases: both captive and possible wild hybrids between two ostrich taxa (Struthio camelus x Struthio molybdophanes), captive hybrids between Lesser Rhea (Rhea pennata) and Greater Rhea (Rhea americana), and two cases involving kiwi species (Apteryx australis x Apteryx mantelli and Apteryx rowi x Apteryx owenii). Interestingly, most reported hybridisation events are linked to human activities such as captive breeding and species translocations, suggesting that natural hybridisation among ratites is exceptionally rare. Nevertheless, the possibility of historical introgression cannot be excluded. Future comparative genomic and ancient DNA studies may reveal past hybridisation events, thereby provide new insights into the evolutionary history of this unique avian lineage.