This bird order comprises most of the diurnal birds of prey, such as hawks, eagles, and vultures.Traditionally, they were included with the falcons in the Falconiformes, but some authorities have decided to split them.
Despite the fact that 13 species hybridize in the wild, there are no thorough studies on Accipiter. Two papers discuss the morphological and behavioural characteristics of two hybrids, namely A. novaehollandiae x fasciatus in Australia (Olsen & Olsen, 1985) and A. badius x A. brevipes in Israel (Yosef, Helbig & Clark, 2001). A study of two African species (A. tachiro and A. toussenelii) found no evidence for hybridization (Louette, 2003).
Seven Aquila species hybridize with at least one other species. There is, however, one species pair that has received most attention: Lesser Spotted Eagle (A. pomarina) and Greater Spotted Eagle (A. clanga). The vulnerable status of the latter species links this situation to conservation issues. Hybrids and backcrosses can be identified based on a number of morphological features, such as nape patch and plumage index (Vali & Lohmus, 2004). Also, hybrid individuals might attain immature characteristics (Vali, 2010). Genetic analyses revealed extensive introgression and asymmetrical hybridization which could be attributed to mate choice based on size differences (Helbig et al., 2005; Vali et al., 2010). Introgression of Z-linked loci proved to be lower compared to autosomal loci (Backstrom & Vali, 2011). The two species use different habitats (Lohmus & Vali, 2005), but anthropogenic habitat alteration might favour hybridization (Maciorowski & Mirski, 2014). In the Biebrza River Vally (Poland), the proportion of broods producing hybrids increased by over 30% between 1996 and 2012 (Maciorowski, Mirski & Vali, 2015). Lastly, one study mapped the migration route of a hybrid individual, which followed a typical A. clanga migration strategy (Meyburg & Meyburg, 2007).
Similar to Accipiter, numerous species (nine) engage in hybridization, but there are no thorough studies of specific cases. Based on morphological and molecular analyses a museum specimen from Louisiana was identified as B. swainsoni x lagopus (Clark & Witt, 2006). Hybrids between B. swainsoni and B. jamaicensis were identified using genetic tools (Hull et al., 2007). Finally, Elorriaga and Munoz (2013) described the interbreeding of B. rufinus and B. buteo in Gibraltar (Spain).
In Southern Siberia, Eastern Marsh Harrier (C. spilonotus) and Western Marsh Harrier (C. aeruginosus) hybridize. The situation was extensively described by Fefelov (2001). More Harrier species are known to interbreed, but they have not been studied in detail.
Allozyme analyses of Black Kite (M. migrans) and Red Kite (M. milvus) revealed low genetic distances, in contrast to their divergent phenotypes. This observation might be attributed to hybridization (Schreiber, Stubbe & Stubbe, 2000).
Backstrom, N. & Vali, U. (2011). Sex- and species-biased gene flow in a spotted eagle hybrid zone. Bmc Evolutionary Biology 11.
Clark, W. S. & Witt, C. C. (2006). First known specimen of a hybrid Buteo: Swainson’s Hawk (Buteo swainsoni) x Rough-legged Hawk (B-lagopus) from Louisiana. Wilson Journal of Ornithology 118, 42-52.
Elorriaga, J. & Munoz, A. R. (2013). Hybridisation between the Common Buzzard Buteo buteo buteo and the North African race of Long-legged Buzzard Buteo rufinus cirtensis in the Strait of Gibraltar: prelude or preclude to colonisation? Ostrich 84, 41-45.
Fefelov, I. V. (2001). Comparative breeding ecology and hybridization of Eastern and Western Marsh Harriers Circus spilonotus and C-aeruginosus in the Baikal Region of Eastern Siberia. Ibis 143, 587-592.
Helbig, A. J., Seibold, I., Kocum, A., Liebers, D., Irwin, J., Bergmanis, U., Meyburg, B. U., Scheller, W., Stubbe, M. & Bensch, S. (2005). Genetic differentiation and hybridization between greater and lesser spotted eagles (Accipitriformes : Aquila clanga, A-pomarina). Journal of Ornithology 146, 226-234.
Hull, J. M., Savage, W., Smith, J. P., Murphy, N., Cullen, L., Hutchins, A. C. & Ernest, H. B. (2007). Hybridization among buteos: Swainson’s hawks (Buteo swainsoni) x red-tailed hawks (Buteo jamaicensis). Wilson Journal of Ornithology 119, 579-584.
Lohmus, A. & Vali, U. (2005). Habitat use by the Vulnerable greater spotted eagle Aquila clanga, interbreeding with the lesser spotted eagle Aquila pomarina in Estonia. Oryx 39, 170-177.
Louette, M. (2003). Size, plumage, moult and supposed hybrids of African Goshawks (Accipiter tachiro/toussenelii group) in DR Congo. Ostrich 74, 18-29.
Maciorowski, G. & Mirski, P. (2014). Habitat alteration enables hybridisation between Lesser Spotted and Greater Spotted Eagles in north-east Poland. Bird Conservation International 24, 152-161.
Maciorowski, G., Mirski, P. & Vali, U. (2015). Hybridisation dynamics between the Greater Spotted Eagles Aquila clanga and Lesser Spotted Eagles Aquila pomarina in the Biebrza River Valley (NE Poland). Acta Ornithologica 50, 33-41.
Meyburg, B. U. & Meyburg, C. (2007). Post-fledging behavior and outward migration of a hybrid Greater X Lesser Spotted Eagle (Aquila clanga X A. Pomarina). Journal of Raptor Research 41, 165-170.
Olsen, P. & Olsen, J. (1985). A natural hybridization of the Brown Goshawk Accipiter fasciatus and Grey Goshawk Accipiter novaehollandiae in Australia, and a comparison of the two species. Emu 85.
Schreiber, A., Stubbe, M. & Stubbe, A. (2000). Red kite (Milvus milvus) and black kite (M-migrans): minute genetic interspecies distance of two raptors breeding in a mixed community (Falconiformes : Accipitridae). Biological Journal of the Linnean Society 69, 351-365.
Vali, U. (2010). Successful breeding of a ten-year-old hybrid spotted eagle Aquila clanga x A. pomarina retaining immature plumage characters. Ardea 98, 235-241.
Vali, U., Dombrovski, V., Treinys, R., Bergmanis, U., Daroczi, S. J., Dravecky, M., Ivanovski, V., Lontkowski, J., Maciorowski, G., Meyburg, B. U., Mizera, T., Zeitz, R. & Ellegren, H. (2010). Widespread hybridization between the Greater Spotted Eagle Aquila clanga and the Lesser Spotted Eagle Aquila pomarina (Aves: Accipitriformes) in Europe. Biological Journal of the Linnean Society 100, 725-736.
Vali, U. & Lohmus, A. (2004). Nestling characteristics and identification of the lesser spotted eagle Aquila pomarina, greater spotted eagle A. clanga, and their hybrids. Journal of Ornithology 145, 256-263.
Yosef, R., Helbig, A. & Clark, W. S. (2001). An intrageneric Accipiter hybrid from Eilat, Israel. Sandgrouse 23, 141-144.