Probing the Puzzling Plumage Patterns of White Wagtails

How can we explain plumage patterns in white wagtails subspecies?

Wagtail taxonomy is a mess. Numerous subspecies have been described based on morphological differences, but they are not supported by genetic data. A recent study in Journal of Evolutionary Biology took another look at several subspecies of the White Wagtail (Motacilla alba). Could they explain the mismatch between plumage and genetics?

 

Six Subspecies

Georgy Semenov and his colleagues sampled six of the nine recognized subspecies – alba, personata, baicalensis, ocularis, lugens and leucopsis – and sequenced 17 microsatellites. In line with previous studies, the genetic analyses revealed little population structure and weak divergence among the subspecies.

Distribution of White Wagtail (Motacilla alba) subspecies (from http://www.wikiwand.com/en/White_wagtail)

 

Puzzling Plumage Patterns

How can ornithologists explain this peculiar pattern of clear morphological differences without genetic differentiation? Recent genomic studies have shown that a small fraction of the genome can underlie such plumage variation (see for example crows and warblers). Something similar might explain the plumage patterns in White Wagtail subspecies.

A white wagtail by the water (from: http://www.hbw.com/)

 

The authors of the current study speculate that a small toolkit of genes might have been shuffled around by hybridization, resulting in the different wagtail head patterns. These patterns are confined to a small number of patches – throat, back and sides of the head and neck – which can be either black or grey. Think shuffling a deck of cards and randomly extracting a combination of cards: black throat, grey back, black on the sides. Hey, that combination looks like personata! (Try it yourself, pick three random colors for each patch and see which subspecies you end up with)

This idea is actually supported by some keen observations. For example, the subspecies persica (from Iran) resembles a certain hybrid between alba and personata (read more about these hybrids here). Similarly, the Moroccan subspecies subpersonata looks like a alba x personata hybrid with the eye-stripe of a lugens x ocularis pair. Suddenly, this is starting to make more sense…

The subspecies persica looks surprisingly similar to a hybrid between personata and alba (from: http://commons.wikimedia.com/)

 

References

Semenov, G.A., Koblik, E.A., Red’Kin, Y.A. & Badyeav, A.V. (2018) Extensive phenotypic diversification coexists with little genetic divergence and lack of population structure in the White Wagtail subspecies complex (Motacilla alba). Journal of Evolutionary Biology

 

The paper has been added to the Motacillidae page.