The order Strigiformes houses about 200 species of owls. Despite their mostly nocturnal lifestyle, several hybrids have been reported.
Two subspecies of the Northern Saw-whet Owl (A. acadicus) might be the outcome of heteropatric speciation, where populations are allopatric and sympatric during different times of the year. There was some gene flow between a sedentary island population (brooksi) and a migratory mainland population (acadicus) in North America (Withrow et al. 2014; Winker et al., 2019).
In 1986, the last female of the Norfolk Island Boobook Owl (N. novaeseelandiae undulata) was crossed with several males of the closely related New Zeeland subspecies (N. n. novaeseelandiae) in order to conserve this endangered subspecies (Norman, Olsen & Christidis, 1998). This intervention led to the debate whether the Norfolk Island Boobook Owl (which now exists as a hybrid population) should be considered extinct or critically endangered (Garnett et al., 2011).
Eastern (O. asio) and Western Screech-owls (O. kennicottii) occasionally hybridize. Morphological analysis showed that sympatric populations display greater differences in bill-length compared to allopatric ones, which points to character displacement (Gehlbach, 2003).
The Spotted Owl (S. occidentalis) is divided into three subspecies, the Northern (S. o. caurina), Mexican (S. o. lucida) and California (S. o. occidentalis) Spotted Owl (Barrowclough, Gutierrez & Groth, 1999). Hybridization occurs between the Northern and Mexican subspecies and between the Northern and California subspecies (Funk et al., 2008). The latter hybrid zone has been studied in detail (Barrowclough et al., 2005; Barrowclough et al., 2011). In the contact zone, more than half of the individuals showed evidence of hybrid ancestry. The distribution of admixed owls suggests that the hybrid zone is moving north (Miller et al., 2017).
Range expansion of Barred Owls (S. varia) has led to hybridization with the endangered California Spotted Owl (S. o. caurina) in western North America (Hamer et al., 1994; Kelly & Forsman, 2004), which was confirmed by genetic analyses (Funk et al., 2007; Haig et al., 2004).
In central Europe two subspecies of the Barn Owl (T. alba) interbreed, namely the white (T. a alba) and reddish-brown (T. a. guttata) forms (Mátics & Hoffmann, 2002). Based on the different ratios of subspecies across Europe, Matics et al. (2005) predicted higher genetic variation in Switzerland compared to Hungary. This hypothesis was confirmed using RAPD markers. ABC modelling showed that the colour cline is the result of expansion from a glacial refugium, followed by natural selection (Antoniazza et al., 2014). Barn Owls seemed to have colonized western Europe in a ring around the Mediterranean. At a secondary contact zone in Greece there is evidence for limited genetic exchange. This pattern is reminiscent of ring species (Burri et al., 2016).
Antoniazza, S., Kanitz, R., Neuenschwander, S., Burri, R., Gaigher, A., Roulin, A. & Goudet, J. (2014). Natural selection in a postglacial range expansion: the case of the colour cline in the European barn owl. Molecular Ecology 23, 5508-5523.
Barrowclough, G. F., Groth, J. G., Mertz, L. A. & Gutierrez, R. J. (2005). Genetic structure, introgression, and a narrow hybrid zone between northern and California spotted owls (Strix occidentalis). Molecular Ecology 14, 1109-1120.
Barrowclough, G. F., Gutierrez, R. J. & Groth, J. G. (1999). Phylogeography of spotted owl (Strix occidentalis) populations based on mitochondrial DNA sequences: Gene flow, genetic structure, and a novel biogeographic pattern. Evolution 53, 919-931.
Barrowclough, G. F., Gutierrez, R. J., Groth, J. G., Lai, J. E. & Rock, D. F. (2011). The Hybrid Zone between Northern and California Spotted Owls in the Cascade-Sierran Suture Zone. Condor 113, 581-589.
Burri, R., S. Antoniazza, A. Gaigher, A. L. Ducrest, C. Simon, L. Fumagalli, J. Goudet and A. Roulin (2016). The genetic basis of color‐related local adaptation in a ring‐like colonization around the Mediterranean. Evolution 70(1): 140-153.
Funk, W. C., Forsman, E. D., Mullins, T. D. & Haig, S. M. (2008). Introgression and dispersal among spotted owl (Strix occidentalis) subspecies. Evolutionary Applications 1, 161-171.
Funk, W. C., Mullins, T. D., Forsman, E. D. & Haig, S. M. (2007). Microsatellite loci for distinguishing spotted owls (Strix occidentalis), barred owls (Strix varia), and their hybrids. Molecular Ecology Notes 7, 284-286.
Garnett, S. T., Olsen, P., Butchart, S. H. M. & Hoffmann, A. A. (2011). Did hybridization save the Norfolk Island boobook owl Ninox novaeseelandiae undulata? Oryx 45, 500-504.
Gehlbach, F. R. (2003). Body size variation and evolutionary ecology of eastern and western screech-owls. Southwestern Naturalist 48, 70-80.
Haig, S. M., Mullins, T. D., Forsman, E. D., Trail, P. W. & Wennerberg, L. (2004). Genetic identification of Spotted Owls, Barred Owls, and their hybrids: Legal implications of hybrid identity. Conservation Biology 18, 1347-1357.
Hamer, T. E., Forsman, E. D., Fuchs, A. D. & Walters, M. L. (1994). Hybridization between Barred and Spotted Owls. Auk 111, 487-492.
Kelly, E. G. & Forsman, E. D. (2004). Recent records of hybridization between Barred Owls (Strix varia) and Northern Spotted Owls (S-occidentalis caurina). Auk 121, 806-810.
Mátics, R. & Hoffmann, G. (2002). Location of the transition zone of the Barn Owl subspecies Tyto alba alba and Tyto alba guttata (Strigiformes: Tytonidae). Act. Zool. Cracov 45, 245-250.
Matics, R., Varga, S., Opper, B., Klein, A. K., Horvath, G., Roulin, A., Putnoky, P. & Hoffmann, G. (2005). Partitioning of genetic (Rapd) variability among sexes and populations of the Barn Owl (Tyto alba) in Europe. Journal of Raptor Research 39, 142-148.
Miller, M. P., T. D. Mullins, E. D. Forsman and S. M. Haig (2017). Genetic differentiation and inferred dynamics of a hybrid zone between Northern Spotted Owls (Strix occidentalis caurina) and California Spotted Owls (S. o. occidentalis) in northern California. Ecology and Evolution.
Norman, J., Olsen, P. & Christidis, L. (1998). Molecular genetics confirms taxonomic affinities of the endangered Norfolk Island Boobook Owl Ninox novaeseelandiae undulata. Biological Conservation 86, 33-36.
Winker, K., Glenn, T.C., Withrow, J., Sealy, S.G., & Faircloth, B.C. (2019). Speciation despite gene flow in two owls (Aegolius ssp.): Evidence from 2,517 ultraconserved element loci. The Auk, 136(2):ukz012.
Withrow, J. J., S. G. Sealy and K. Winker (2014). Genetics of divergence in the Northern Saw-whet Owl (Aegolius acadicus). The Auk 131(1): 73-85.