Birder's Guide

OCT 2016

Birder's Guide is the American Birding Association's newest publication. Each issue focuses on a key subject, providing tips from experienced birders on a wide variety of topics like Travel, Listing & Taxonomy, Gear, and Conservation & Community.

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23 October 2016 | Birder's Guide to Listing & Taxonomy Seattle, Washington temnurus@gmail.com Alan Knue n Although the AOU NACC and Clements lump it with Mallard, the IOC (International Ornithological Congress) treats Mexican Duck (shown here) as a species. HBW differs from both by treating it as a subspecies of Mottled Duck. Photo © E.J. Peiker (allopatric populations) and (2) how to interpret hybridization between populations. As Churchill (2014) pointed out, allopatric populations are problematic for the BSC because one cannot test what would naturally happen should these populations come into contact. In the more distant past, treatment of these forms was subject more to opinion than good evidence. That's where modern research techniques, such as studies focusing on vocalizations and genetics, contribute greatly to sorting out just how dis- tinctive these populations are and whether they should be lumped or split. For example, the distinctive western subspe- cies (elegans) of Red-shouldered Hawk is sepa- rated geographically from the other subspecies of Red-shouldered Hawk by thousands of miles. (The closest ones are in central Texas.) It is dis- tinctive in both adult and immature plumages and is about 10% smaller than the eastern birds. In the past, elegans earned the ranking of a well- differentiated subspecies because it was believed that these differences were not enough to allow the populations to function as separate species. It has since been demonstrated that elegans is well divergent in both mitochondrial and nuclear DNA (Hull et al., 2008). Could this be the evi- dence needed to elevate elegans to separate spe- cies status in a future supplement of the Check- list? Perhaps… That brings us to populations that interbreed. The NACC's preface states that the "essential lack of free interbreeding rather than complete re- productive isolation has been and continues to be the fundamental operating criterion for spe- cies status by workers adhering to the BSC". At

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