[Maine-birds] Marsh Sparrows: Nelson's & Saltmarsh and hybrids

Birders visiting Maine, especially Scarborough marsh, to see both species of Marsh Sparrows—Nelson's and Saltmarsh—know they face an identification challenge. Not only are the two similar and not easy to see, but in the most heavily birded marshes there are hybrids. There is an assumption that these hybrids show mixed traits. But are we right about that and can such hybrids be weeded out in the field?

The latest research on this topic was published this week in "The Auk: Ornithological Advances." Here is a pertinent quote: "appearance 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." (http://aoucospubsblog.org/2015/06/17/species-lines-blur-between-two-sparrows-in-new-englands-tidal-marshes/ — access to the paper itself is linked from this page)

The taxa appear to act as good species, and fewer than expected recent generation hybrids (F1, F2) were found in the hybrid zone, roughly from Weskeag south to northeastern Massachusetts. In that hybrid zone, however, 52% of birds sampled were of mixed genetic background. From the birder's perspective, the chief characters we use to identify birds are fairly good IF one lumps Nelson's and Nelson's backcrosses together versus Saltmarsh and Saltmarsh backcrosses. By doing this, however, we underestimate the number of actual hybrids encountered. From a conservation perspective, it would be good to know how extensively these species interbreed, and determining that in the field without DNA appears not possible with presently known characters.

Louis Bevier
Fairfield

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