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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Page 23 of 61

To Be a Species , or Not to Be a Species To Be a Species , or Not to Be a Species The answer to the question "What is a species?" is of great interest to birders because many of us make lists, use field guides, and attempt to name every bird we see in the field. 22 Birder's Guide to Listing & Taxonomy | October 2016 his ability to name birds is based on years of cumulative observation, data, study, and research, all of which has resulted in various lists of currently recognized species. The ABA bases its checklist of birds on the American Ornithologists' Union (AOU) Check-list of North American Birds, which is pro- duced by the AOU's Committee on Classifica- tion and Nomenclature of North and Middle American Birds (a.k.a. the "North American Classification Committee" or NACC). The NACC is tasked with maintaining this list and updating it, usually annually, drawing on pro- posals based on evidence-based research. This annual update is like Christmas for many bird- ers, because it often contains gifts in the form of newly recognized species. Why does the list change? Species determi- nation depends on the accumulation of evi- dence, and our knowledge base increases every year. Also, answering the question "What is a species?" isn't straightforward. Species con- cepts are human constructs. We view species formation as a snapshot in time. But evolution doesn't proceed like clockwork with all species formation happening on the same timetable. Some species pairs are quite divergent with no mixing, while others have only recently di- verged with limited hybridization at their range T boundaries. Still other pairs consist of two dis- tinctive "bookends", with every conceivable combination of traits in between. The boundar- ies of some species pairs will always look a bit blurred and will not easily fit into any system of rules for determining species limits. We at least know that the NACC uses the biological species concept (BSC) as the basis for inclusion of species on the Check-list (see Churchill, 2014, for a history and discussion of species concepts). In the preface of the seventh edition (AOU, 1998), species "are considered to be genetically cohesive groups of populations that are reproductively isolated from other such groups". Further, "geographic isolation leads to genetic change and potentially to the reproduc- tive isolation of sister taxa. If and when these closely related forms later coexist, reproductive isolating mechanisms, such as distinctive dis- plays and vocalizations, serve to maintain the essential genetic integrity of the newly formed biological species". This is the definition by which all groups of birds are measured, and it works well in most cases. However, some subjective judgment will always be involved when determining species limits according to the BSC. There are two chal- lenges in the application of the BSC: (1) how to treat related populations that do not overlap ?

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