Mixing in the Marshes: King rail and clapper rail hybridize in Virginia

Cryptic introgression between two rail species in Virginia.

Distinguishing between king rail (Rallus elegans) and clapper rail (R. crepitans) is challenging, to say the least. These secretive birds have similar diets, calls and morphology. In general, however, king rails are slightly larger and have a more deep rust-colored plumage compared to clapper rails. Despite their morphological differences, genetic studies indicated that these rails do represent distinct species.

A king rail (from: http://www.allaboutbirds.com/) © Luke Seitz

 

Wetlands

King and clapper rail prefer different kinds of wetland. King rails are mostly found in a diverse range of habitats from freshwater to brackish marshes, whereas clapper rails live exclusively in brackish and saltwater marshes. But where they overlap, they interbreed (as exemplified by the picture below showing a mixed pair). Stephanie Coster (West Virginia University) and her colleagues wanted to know if occasional hybridization results in gene flow between these species. Their results recently appeared in the journal Ecology and Evolution.

The researchers sampled birds across a salinity gradient on the east coast of the US. The genetic analyses – based on mtDNA and 13 nuclear markers (SNPs) – revealed several admixed individuals in Virginia. Most of these birds were backcrosses to clapper rail, indicating that hybrids are fertile.

A mixed pair of clapper rail (left) and king rail. Picture taken by Robert Ostrowski (from: Coster et al., 2018)

 

Wave-front Dynamics

The uncovered genetic patterns fit an evolutionary history proposed by Storrs L. Olson in 1997. He envisioned that an ancestral population of king rails was adapted to freshwater. Birds at the periphery became isolated due to rising sea levels and adapted to salty environments. These birds would evolve into clapper rails. Later on, this new lineage expanded into brackish habitats, thereby displacing the resident king rails.

Initially, the expanding clapper rails are outnumbered by king rails, leading to hybridization. As the expansion proceeds, king rails and previously produced hybrids are engulfed by clapper rails, thereby overturning the numerical imbalance. Consequently, hybrids have a higher chance of backcrossing with clapper tails, resulting in a genetic wake of introgressed genes following the wave-front of the expanding clapper rails.

A clapper rail (from: https://www.audubon.org/)

 

References

Coster, S.S., Welsh, A.B., Costanzo, G., Harding, S.R., Anderson, J.T., McRae, S.B. & Katzner, T.E. (2018) Genetic analyses reveal cryptic introgression in secretive marsh bird populations. Ecology and Evolution.

 

The paper has been added to the Gruiformes page.