Hybridization occurs in all families (Upupidae, Phoeniculidae, and Bucerotidae), except for the Bucorvidae.
There appears to be introgression across hybrid zone between two subspecies of Red-billed Hornbill (Tockus erythrorhynchus rufirostris and T. e. damarensis) in Namibia, based on morphological data (Delport, Kemp & Ferguson, 2004).
One study documented the interbreeding of Great Hornbill (Buceros bicornis) and Rhinoceros Hornbill (B. rhinoceros), using morphological and genetic techniques (Chamutpong et al., 2013).
Du Plessis et al. (2007) refer to “apparent hybridization” between Green Woodhoopoe (Phoeniculus purpureus) and Violet Woodhoopoe (P. damarensis) in their behavioural study.
Chamutpong, S., Ponglikitmongkol, M., Charoennitikul, W., Mudsri, S. & Poonswad, P. (2013). Hybridisation in the Wild between the Great Hornbill (Buceros Bicornis) and the Rhinoceros Hornbill (Buceros Rhinoceros) in Thailand and Its Genetic Assessment. Raffles Bulletin of Zoology 61, 349-358.
Delport, W., Kemp, A. C. & Ferguson, J. W. H. (2004). Structure of an African red-billed hornbill (Tockus erythrorhynchus rufirostris and T-e. damarensis) hybrid zone as revealed by morphology, behavior, and breeding biology. Auk 121, 565-586.
du Plessis, M. A., Simmons, R. E. & Radford, A. N. (2007). Behavioural ecology of the Namibian violet woodhoopoe Phoeniculus damarensis. Ostrich 78, 1-5.