The order Pelecaniformes contains several medium to large waterbirds. Currently, it includes the following families: Pelicans (Pelecanidae), Ibises and Spoonbills (Threskiornithidae), Herons, Egrets and Bitterns (Ardeidae), the Shoebill (Balaenicipitidae), and the Hamerkop (Scopidae).
In Japan, the intergeneric hybridization event between a male Little Egret (E. garzetta) and a female Chinese Pond Heron (Ardeola bacchus) was recorded and described (Mashiko et al., 2012).
In South Dakota, the interbreeding of a Tricolored Heron (E. tricolor) and a Snowy Egret (E. thula) was reported (Meeks et al., 1996). More reports on hybrid herons can be found in the references of these papers, for instance Sprunt (1954).
Several sightings of dark morphs of Little Egret in India might actually be hybrids with Indian Reef Heron (E. schistacea)(Koparde & Yesou, 2017).
A study that set out to investigate whether the Mauritanian Spoonbill (Platalea leucordia balsaci) is a valid subspecies detected introgression from another subspecies, the Eurasian Spoonbill (P. l. leucordia), into P. l. balsaci (Piersma et al., 2012).
Interspecific hybridization of spoonbills has been observed in captivity and nature. The international Zoo Yearbook of 1984, for instance, reports hybridization between a Roseate (Platalea ajaja) and a Eurasian (P. leucorodia) Spoonbill and between an African (P. alba) and a Eurasian Spoonbill.
In South Korea, researchers observed natural hybridization between Black-faced Spoonbill (P. minor) and Eurasian Spoonbill (Kwon et al., 2017).
Populations of Great Cormorants (Phalacrocorax carbo) in the UK consist of two subspecies, namely ‘North Atlantic’ (P.c. carbo) and ‘Eurasian’ (P.c. sinensis) birds, that probably hybridize (Goostrey et al., 1998; Marion & Le Gentil, 2006).
Goostrey, A., Carss, D. N., Noble, L. R. & Piertney, S. B. (1998). Population introgression and differentiation in the great cormorant Phalacrocorax carbo in Europe. Molecular Ecology 7, 329-338.
Koparde, P & Yesou, P (2017) Probable hybrids Little Egret x Indian Reef Heron in India and Sri Lanka. Dutch Birding 39, 238-246.
Kwon, I.-K., Lee, K.-S., Lee, J.-Y., Park, J.-H. & Yoo, J.-C. (2017). Hybridization between the Black-Faced Spoonbill (Platalea minor) and Eurasian Spoonbill (Platalea leucorodia) in South Korea. Waterbirds 40, 77-81.
Marion, L. & Le Gentil, J. (2006). Ecological segregation and population structuring of the cormorant Phalacrocorax carbo in Europe, in relation to the recent introgression of continental and marine subspecies. Evolutionary Ecology 20, 193-216.
Mashiko, M., Fujioka, M., Moriya, K., Hagimoto, T., Yamaguchi, M. & Toquenaga, Y. (2012). Natural Hybridization between a Little Egret (Egretta garzetta) and a Chinese Pond Heron (Ardeola bacchus) in Japan. Waterbirds 35, 160-163.
Meeks, W. A., Naugle, D. E., Johnson, R. R. & Higgins, K. F. (1996). Interbreeding of a tricolored heron and a Snowy Egret in South Dakota. Auk 113, 955-957.
Piersma, T., van der Velde, M., El-Hacen, E. M., Lok, T. & Overdijk, O. (2012). Molecular verification of the subspecies status of the Mauritanian Spoonbill Platalea leucorodia balsaci. Ardea 100, 131-136.
Sprunt, A. (1954). A hybrid between the Little Blue Heron and the Snowy Egret. The Auk, 314-314.