Hybrid Owls: Californian Contacts and Mediterranean Migrants

Who doesn’t like owls? Silently gliding through the night, with their big eyes and flexible neck. And they even hybridize!

A wise old owl sat on an oak;
The more he saw the less he spoke;
The less he spoke the more he heard;
Why aren’t we like that wise old bird?

– Old Nursery Rhyme


When you look up the definition for hybridization in the English Oxford Dictionary, it states that hybridization is “The process of an animal or plant breeding with an individual of another species or variety.” Most people think that hybrids are the outcome of different species interbreeding (such as a lion and a tiger), but the definition also mentions variety. Indeed, hybrids between varieties, often referred to as subspecies, are also possible. Two recent studies on owls emphasize this possibility. Mark Miller (U.S. Geological Survey) and colleagues investigated a hybrid zone between subspecies of Spotted Owl (Strix occidentalis) in northern California. In Europe, Reto Burri (Uppsala University), Sylvain Antoniazza (University of Lausanne) and others focused on color morphs of the Barn Owl (Tyto alba).


A hybrid zone without hybrids

The study of hybridization between Northern Spotted Owls (S. o. caurina) and California Spotted Owls (S. o. occidentalis) has a long history (see the Strigiformes page for an overview of the publications). Among others, George F. Barrowclough and Susan M. Haig have unraveled the interactions between these subspecies. This new study provides a detailed genetic perspective on the hybrid zone in Northern California. 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.

Surprisingly, no first generation (F1) hybrids were detected. Perhaps some hybrid owls were wrongly classified in the analyses or maybe the researchers did not sample F1-hybrids. It could also be that there is no recent hybridization. Several changes in the Californian landscape might have reduced the connectivity between different owl populations. In particular, the authors point to the Fountain Fire of 1992 which occurred at the intersection of the subspecies’ habitat.



A Northern Spotted Owl


Around the Mediterranean

Barn Owls gradually changes from white in Iberia to dark rufous in Northeastern Europe. In this study, researchers show that the rufous coloration is linked to a recent variant of the MC1R gene. More interestingly (from a hybridization point of view), 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, i.e. populations with a ring-like distribution around a geographic barrier that are interconnected by gene flow. In recent years, it became obvious that most classical ring species do not adhere to this strict definition. Can the Barn Owl pass the test?


The putative expansion routes and Greek second contact zone of the Barn Owl (adapted from Burri et al. 2016)




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.

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.


These papers have been added to the Strigiformes page.

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