19/11/2018
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Black-eared Wheatear: two species, not one

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A new study has demonstrated that Eastern and Western Black-eared Wheatears should be treated as separate species.

The research, recently published in Journal of Evolutionary Biology, examined and sequenced the DNA of four black-and-white wheatears: Pied, Cyprus and both Western and Eastern Black-eared Wheatears. This was used to create a species tree, which illustrates the evolutionary relationships between the studied species. This initial analysis established that, despite their superficial similarities, Western and Eastern Black‐eared Wheatears have evolved as independent taxa, meaning that they should be recognised as full species.


Adult male Western Black-eared Wheatear has a more orange-buff back, with a narrower black face mask, when compared to its Eastern counterpart (Mick Temple).

In terms of plumage, black backs and neck-sides separate Pied and Cyprus Wheatears from either Black-eared Wheatear. Interestingly, Western Black-eared Wheatear can be separated from Eastern Black-eared, Pied and Cyprus Wheatears by differences in mitochondrial DNA, yet the latter three species cannot be distinguised individually. Eastern Black-eared Wheatear readily hybridises with Pied Wheatear where their ranges overlap. 

Spring male Western Black-eared Wheatear averages richer, more orange-buff upperparts with a narrower black face 'mask', whereas the upperparts tend to be paler and the black mask more extensive (extending significantly above the eye and bill) in male Eastern Black-eared. Female and first-winter Eastern tend to have darker backs and a richer orange breast than Western; Western appears slightly more uniform, with a shorter primary projection, while tending to show a warmer buff basal colour, more akin to Desert Wheatear. In fact, first-winter Eastern Black-eared Wheatear is more similar in appearance to first-winter Pied than it is Western Black-eared.

Western Black-eared Wheatear is a summer migrant to Iberia, southern France and northern Italy, wintering south of the Sahara, with Eastern Black-eared Wheatear a summer migrant from southern Italy and the Balkans east to Iran. As of the end of 2016, both taxa have occurred as rare but relatively regular vagrants to Britain, with a total of 60 accepted records. It is much rarer in Ireland, with just four documented occurrences.


Although superficially very similar to its Western counterpart, the new research suggests that Eastern Black-eared Wheatear has evolved entirely separately and is a good species in its own right (Duha).

Reto Burri, one of the researchers involved in the study, summarised: "Wheatears often present evolutionary biologists with an intriguing puzzle: species with very similar plumage more often than not are not each other's closes relatives.

"In the case of Pied, Cyprus and Black-eared Wheatears, mitochondrial DNA had already provided clues that plumage colouration might have little to say about the taxa's true relationships. This is now corroborated by our analyses of genome-wide genetic data, which uncovers a species tree with Western Black-eared splitting off first, followed by Pied Wheatear, and Eastern Black-eared and Cyprus Wheatears being sister species.

"For taxonomy this implies that, unless we 'lump' all four taxa into a single species with four distinct subspecies, Western and Eastern Black-eared Wheatear cannot be upheld as subspecies, but need to be regarded as independent species."

 

Reference

Schweizer, M, Warmuth, V, Kakhki, N A, Aliabadian, M, Förschler, M, Shirihai, H, Suh, A, & Burri, R. 2018. Parallel plumage color evolution and introgressive hybridization in wheatearsJournal of Evolutionary Biology. DOI: https://doi.org/10.1111/jeb.13401