Unraveling the history (or histories?) of the Red-bellied Woodpecker

Reconstructing the history of the Red-bellied Woodpecker.

Florida is not only a popular holiday destination, it also houses a famous suture zone. What is a suture zone, you ask? It is a term from phylogeography, the study of the historical processes that culminated in the present distributions of organisms. In 1968, Remington defined a suture zone as “a band of geographic overlap between major biotic assemblages, including some pairs of species or semispecies which hybridize in the zone.” In other words, it is a region that houses numerous contact zones between various organisms.

suture zones

The expansion from Pleistocene refugia (indicated by the letters) let to the formation of numerous secondary contact zones. Regions where multiple contact zones cluster are called suture zones (from: Swenson & Howard 2005 The American Naturalist)

 

Ice Age Legacy

Suture zones are the outcome of post-Pleistocene expansion. During the Pleistocene, most of North America was covered with ice sheets, pushing animals and plants into southern refugia. Once the ice sheets melted, the organisms expanded from their refugia and recolonized North America. In this process, populations from different refugia came into secondary contact (I have written about this before, see here and here). As a consequence, populations on both sides of the contact zone are genetically differentiated.

Avian examples include Carolina Chicadee (Poecile carolinensis) and Barred Owl (Strix varia). Another putative instance of this scenario is the Red-bellied Woodpecker (Melanerpes carolinus). A recent study in The Wilson Journal of Ornithology investigated whether this species conforms to the expected pattern.

red-bellied woodpecker.jpg

A Red-bellied Woodpecker (from: http://www.allaboutbirds.com/)

 

Different Histories

George Barrowclough (American Museum of Natural History) and his colleagues collected samples from across the range of the Red-bellied Woodpecker. Analyses of the mitochondrial gene ND2 revealed striking patterns. Populations outside of Florida housed one common genetic variant (or haplotype) and a few uncommon ones. This genetic distribution indicates an expanding population. Statistical tests, such as Fu’s Fs, supported this conclusion. Populations in Florida, by contrast, showed little genetic variation and tested negative for population expansion.

The authors state that “these alternate haplotype frequencies and associated demographies suggest that the populations have had separate evolutionary histories and are now in secondary contact in a well-known suture zone.”

 

Plumage Patterns

The genetic results are supported by morphological data. The researchers examined 204 adult males. Each individual was scored for the presence or absence of a distinct tan-colored forehead band between the nostrils and the eyes. This plumage pattern is used to diagnose the subspecies perplexus, which occurs in Florida. Mapping the scores for this characteristic on a map revealed a gradual decrease of this band from south to north.

Taken together, the geographic patterns in mtDNA and plumage might lead to the recognition of the Florida populations as a separate species: the Florida Red-bellied Woodpecker. Another reason to book a holiday to Florida if you are an avid bird watcher.

woodpeckers

Specimens of the Florida subspecies perplexus (two on the left) and the nominate subspecies carolinus (two on the right). Notice the tan-colored forehead on the perplexus individuals (from Barrowclough et al. 2018, The Wilson Journal of Ornithology)

 

References

Barrowclough, G.F., Groth, J.G., Bramlett, E.K., Lai, J.E. & Mauck; W.M. (2018) Phylogeography and geographic variation in the Red-bellied Woodpecker (Melanerpes carolinus): characterization of mtDNA and plumage hybrid zones. The Wilson Journal of Ornithology, 130(3): 671-683.

 

This paper has been added to the Piciformes page.

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