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Think You Know Sharp-tailed Sparrows? Think Again

In 1995, the AOU formally split Sharp-tailed Sparrow into Nelson’s and Saltmarsh Sparrow based on differences in “song, morphology and habitat”. From the standpoint of birders, the split was pretty straight forward. the two species are mostly distinguishable in the field, Nelson’s with it’s buffy breast and Saltmarsh with its fine streaks (Saltmarsh is Streaky, as I was taught). The AOU also paid heed to the hybrid question, as well, stating that the species had:

 limited interbreeding at a secondary contact zone in southern Maine. Although Rising and Avise (1993) suggested retaining these two taxa as subspecies of caudacutus, they do not interbreed freely and should be ranked at the species level.

A great rundown of the science used to formalize can be found at this older post at Biological Ramblings.

A classic long-billed, streaky breasted Saltmarsh Sparrow, photo by nebirdsplus via flickr

A classic long-billed, streaky breasted Saltmarsh Sparrow, photo by nebirdsplus via flickr

It turns out that the last line may have been a bit premature. A recent study soon to be published in The Auk: Ornithological Advances, suggests that the line between Saltmarsh and Nelson’s Sparrows may be a lot less firm than we’ve been assuming. That small hybrid zone is apparently like a Sharp-tailed Sparrow thunderdome. Anything goes. And the kicker is that, unlike a number of common hybrid pairings in North America, the birds that result from this hybridization can be impossible to identify in the field.

[A]ppearance alone is not enough to identify these hybrid zone birds: there is no single, intermediate “phenotype” or physical appearance common to all of the first-generation hybrids found, and birds from further backcrossed generations were often indistinguishable from the parent species. Fifty percent of birds identified as “pure” Nelson’s or Saltmarsh Sparrows in the field turned out be the descendants of hybrids when their DNA was analyzed.

It turns out that it would be a mistake for birders to assume that we would be able to recognize a Sharp-tailed Sparrow of hybrid parentage, particularly in those areas where the two birds share a wintering range. It’s not only problematic for birders looking to accurately census a saltmarsh or add either of the specie to their list, but for conservation interests as well. Both the species rely on saltmarsh habitat, which is under threat from both land and sea in the form of encroaching development and rising sea levels.

For more information on this fascinating study soon to be published, see the Auk’s Publication Blog.

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