Commentary: Why is Hybridization So Uncommon in Old World and New World Vultures?

The incidence of hybridization varies across the avian phylogeny. For some bird orders numerous hybrid records have been reported, whereas hybridization seems to be a rare phenomenon in other bird orders. Vultures are an example of a bird group with few hybrid records, raising the question why hybridization is so uncommon in this group of birds. I searched the scientific literature for hybrid records in both Old World Vultures (part of the family Accipitridae) and New World Vultures (family Cathartidae). My search uncovered only three published cases of natural hybrids in the genus Gyps: White-backed Vulture (Gyps africanus) × Cape Vulture (Gyps coprotheres), Rüppell’s Vulture (Gyps rueppelli) × Griffon Vulture (Gyps fulvus), and Rüppell’s Vulture × Cape Vulture. However, these three cases require genetic confirmation before they can be deemed reliable. Several captive hybrids have also been documented in European zoos, but these hybrids probably represent exceptional cases. The rarity of wild vulture hybrids might be related to their relatively old evolutionary age, although the exact relationship between hybridization probability and genetic divergence remains to be determined. In addition, strong prezygotic barriers, such as species-specific aerial displays and strong social bonds, might play a role in preventing the production of hybrids. This exercise showed how collecting and analyzing hybrid records in less hybridization-prone groups can help to understand why hybridization is rare in these bird groups.