Three color morphs of the Reunion Grey White-eye meet in three narrow contact zones.
Reunion is a small French island, situated east of Madagascar. It houses an endemic bird species, the Reunion Grey White-eye (Zosterops borbonicus), of which three plumage morphs interbreed. All the more reason to visit this island!
Three Little Brown Birds
The three color morphs mainly differ in head coloration. Across the island you can find a brown-headed brown morph (BHB), a grey-headed brown form (GHB), and a brown-naped brown form (BNB). These populations meet in three contact zones across the island. Boris Delhaie and his colleagues explored these contact zones using cline theory (if you want to know more about cline analyses, check out my previous post here) based on plumage characteristics and microsatellites.
The analyses revealed very narrow hybrid zones that coincide with several natural physical barriers, such as dry river beds and lava streams. However, the location of the hybrid zones is shifted compared to the steep changes in vegetation and climatic variables. This suggests that the hybrid zones stabilized on these barriers but are not directly associated with them. Probably, the three color morphs diverged in allopatry and later met at the physical barriers.
No Genetic Structure
The plumage characters showed very steep transitions across the contact zones, indicating that they might be important in mate choice. The genetic markers, in contrast, showed no structure. This observation can be explained in two ways: (1) recent origin of these color morphs, or (2) genetic differentiation is limited to a few genomic regions. Unfortunately, the resolution of microsatellites is too weak to discriminate between these alternatives. As so often, genomic data is warranted here.
Delahaie, B., Cornuault, J., Masson, C., Bertrand, J.A., Bourgeois, Y.X., Milá, B., Thébaud, C., (2017) Narrow hybrid zones in spite of very low population differentiation in neutral markers in an island bird species complex. Journal of Evolutionary Biology. http://onlinelibrary.wiley.com/doi/10.1111/jeb.13177/abstract
This paper has been added to the Zosteropidae page.