Splitting the Sage Sparrow?

Sage Sparrow, California Sage Sparrow, or Great Basin Sage Sparrow?   Photographed by Jamie Chavez in Santa Barbara County, California, and shared under Creative Commons license.

Talk of splitting the Sage Sparrow has been circulating for some time, and according to the just-released list of proposals being considered by the American Ornithologists Union in 2013, this might be the year it actually happens.

The idea of splitting what we currently know as the Sage Sparrow has been around since the late 1800s.  In 1887, Robert Ridgway said that Sage Sparrow included two species, "with scarcely any doubt."  In 1889, Grinnell collected adults and fledged young of two different subspecies at the same location in southern California and agreed that they should be considered two different species.  However, some populations seemed intermediate and so since that time, most ornithologists have considered Sage Sparrows to be one species.  Recently, as with many questions of splitting or lumping, the tide has been shifting back towards splitting.  For example, Beadle and Rising's "Sparrows of the United States and Canada: The Photographic Guide" (2003) considers them to be two species as well.

There are five recognized subspecies of Sage Sparrow, four of which occur in the United States and one of those four is limited to San Clemente Island off the coast of California.  The proposed split would remove the widespread inland subspecies, "nevadensis" from the other four subspecies, and proposes to call the coastal four subspecies "California Sage Sparrow" and the interior nevadensis, "Great Basin Sage Sparrow."

Should the Sage Sparrow be split?  Many people think so, and they have good reason to.  First, there are differences in appearance between the proposed new species.  The "Great Basin Sage Sparrow" is paler than the "California Sage Sparrow," with more distinct streaks on its back.  The Great Basin Sage Sparrow is less well marked on the face, with a thinner and fainter malar, and it has more streaking on the flanks and breast.  The Great Basin Sage Sparrow is also larger than the California Sage Sparrow, and has white on the edges of the tail feathers, which is generally lacking in the California Sage Sparrow.  These differences are clearest in individuals away from the area of overlap in central California, interior nevadensis and coastal belli, but the subspecies in central California, canescens, are intermediate in appearance.  Some great photos for comparison are here (belli), here (canescens), and here (nevadensis).

Second, there are differences in songs, and there is a great post already written elsewhere by Walter Szeliga on that topic.  There's also a great range map of three subspecies in that post.

Genetic relationships between Sage Sparrow subspecies, from Johnson & Marten 1992.  The top two "brackets" would collectively be the California Sage Sparrow, and the lower bracket would be the Great Basin Sage Sparrow.

Third, there are significant genetic differences between the subspecies that correspond to their external appearances.  The figure above shows that the interior "Great Basin Sage Sparrow" (A. b. nevadensis) stands as a group on its own.  This figure is based on allozymes, a relatively early form of genetic data that has since been largely replaced by newer methods.  Recently, newer genetic methods have shown the same patterns.  The map below shows types of mitochondrial DNA across the areas where these putative species meet, and shows that there is little overlap between the orange nevadensis types and the blue (belli) and yellow (canescens) types.

Sage Sparrow mitochondrial haplotypes from Cicero & Koo 2012.  Note the narrow range of overlap between interior nevadensis (orange) and canescens (yellow).

All of this morphological, behavioral, and genetic data makes a decent case for splitting the species.  However, the real key to making the determination of whether any species should be split is to determine how reproductively isolated the two candidate species are.  A small amount of mixing is okay (some very clear species hybridize ocassionally), but there should be good evidence that interbreeding is limited.  The genetics data above gives a good indication of this: it usually requires very little interbreeding to cause genes to mix extensively, so the narrow zone of mixing is evidence for some reproductive isolation.  We'll have to wait a few months to see what the AOU decides, but it seems likely that Sage Sparrows will be split in the very near future.