The Hybrid Zone

Saltmarsh Sparrows (shown) produce hard-to-identify hybrids with Nelson’s Sparrows. Image credit: K. Papanastassiou

Saltmarsh Sparrows (shown) produce hard-to-identify hybrids with Nelson’s Sparrows. Image credit: K. Papanastassiou

Picture, if you will, a coastal marsh in New England where closely related sparrow species — the Saltmarsh Sparrow and Nelson’s Sparrow— interbreed where their ranges overlap. The birds produce hybrid offspring that can backcross with either parent species, until a large percentage of mixed-species birds forms in the area.

You’ve just crossed into the hybrid zone.

Beyond TV science fiction, Kate O’ Brien, a biologist with the U.S. Fish and Wildlife Service’s Rachel Carson National Wildlife Refuge, has helped author a recent study of hybrid zones with lead author Jennifer Walsh and Adrienne Kovach of the University of New Hampshire, Gregory Shriver of the University of Delaware, and Brian Olsen of the University of Maine. The five are members of the Saltmarsh Habitat & Avian Research Program, a collaboration of academic, government, and nonprofit researchers focused on the conservation of tidal marsh birds.

Both the Saltmarsh Sparrow and Nelson’s Sparrows are considered high priorities for conservation in the region, and the Saltmarsh Sparrow in particular is considered globally vulnerable to extinction. O’Brien says to ensure both species have a secure future, the first step is making sure scientists know for certain which is which.


“Even given the significant challenges the Saltmarsh Sparrow faces, it is a fortunate species to be the focus of numerous talented researchers in the SHARP community,” O’Brien says. “It is a species that has gone from relative obscurity to one of intense focus.  Only by learning more about the species will we be able to monitor, manage and conserve it.”  

O’Brien and the other researchers found that appearance alone is not enough to identify these hybrid zone birds, and that birds from further backcrossed generations were often indistinguishable from the parent species. Fifty percent of birds identified as pure Saltmarsh or Nelson’s Sparrows in the field turned out be the descendants of hybrids when their DNA was analyzed.


Nelson’s Sparrow. Image credit: Andy Reago and Chrissy McClarren.

They examined birds in the hybrid zone on the coast of Maine, New Hampshire, and Massachusetts. Each bird was classified based on its appearance as a Saltmarsh Sparrow, Nelson’s Sparrow, or a hybrid, and then a blood sample was taken so that the accuracy of this identification could be confirmed with DNA.  The genetic data was compared with data the researchers collected on plumage, bill size, and body size to determine if physical traits could be used to predict genetic species and hybrids. While the physical traits could not distinguish pure species from hybrids, they could reliably separate birds with primarily Saltmarsh Sparrow gene pools from those with primarily Nelson’s Sparrow gene pools.

These findings have important implications given the restricted range of the Saltmarsh Sparrow. Currently about 150 miles (or 15 percent) of the range falls within the hybrid zone, where both species occur, while mixed individuals occur well beyond the boundaries of this zone. Since individuals thought to be either “pure” Saltmarsh or “pure” Nelson’s may frequently carry hybrid genes, the researchers suggested DNA identification may be required to ensure accurate monitoring of the degree of hybridization and the population status of Saltmarsh and Nelson’s Sparrows within this area. The jury is still out, however, on the significance of hybridization — particularly when it comes to adapting to climate change.  Some scientists theorize that novel gene combinations may result in increased adaptation and increased resilience to climate change, while others suggest they could result in decreased survival of hybrids. Only time — and future research — will tell.  

Learn more about this research here.

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