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Sibley’s field identifiable subspecies

David Sibley’s ground-breaking Guide to Birds, 11 years old this year and still the standard by which North American field guides are judged, was notable for his treatment of identifiable subspecies as regional forms rather than the seemingly more exact latin named subspecies.  This was done largely to discourage birders from jumping the gun and making ill-informed identifications to specific subspecies (which can be really tough even when they’re easy), but also to encourage observers to be more aware of the incredible variation within species so as not to overwhelmed by an aberrant individual.   Sibley’s explanation, in his own words, is here.

But subspecific identification is a siren song, and the lure is strong, and David Sibley himself has offered a list of subspecies that he considers to identifiable in the field.  His rationale for what he includes, and doesn’t include, is as follows:

In trying to establish consistent and simple (even if subjective) criteria for naming subspecific variations, I have approached this list narrowly from a birder’s field identification perspective. For each species I asked the question “If I were on an island in the middle of the ocean, and one [species x] showed up, would I be able to tell which regional population it came from?” If the answer is a confident “yes”, it gets a name. I’ve also included names for a lot of “maybe” subspecies, and for these I describe my concerns in the comments.

This is not a list of “identifiable forms”. I have limited this list to subspecies, which have a discrete geographic range. Color morphs, age and sex variations, hybrids, and more can be fascinating and there is often scientific value in recording those details, but I have excluded them from this list.

Birders may quibble with what Sibley includes – and what he doesn’t include – but a full accounting of the identifiable subspecies and, in fact, the determination of what it even really means to be a subspecies, is ongoing.

It’s fascinating stuff, and if the prevalence of “more study needed” in this list is any indication, these questions are going to keep interested field birders busy for some time to come.

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Nate Swick

Nate Swick

Editor, Social Media Manager at American Birding Association
Nate Swick is the editor of the American Birding Association Blog, social media manager for the ABA, and the host of the American Birding Podcast. He lives in Greensboro, North Carolina, with his wife, Danielle, and two young children. He is the author of Birding for the Curious and The ABA Field Guide to Birds of the Carolinas.
Nate Swick

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  • “…the determination of what it even really means to be a subspecies, is ongoing”

    I think that’s a key phrase… Even the term “species” is far more difficult to define and use consistently within biology than many realize. The term “subspecies” is far murkier, especially when so often it is only construed in terms of what the human visual system notices. Determination by genetics is better, but still somewhat crude and questionable. Maybe there are 6 subspecies of say Northern Cardinal, if only we had enough precise detailed information on enough populations… or, maybe most subspecies currently recognized, lack any significant or real biological meaning, and are merely defined into existence.

  • I can’t believe the Sibley guide is 11 years old! I applaud Sibley’s effort to be objective and totally transparent in presenting his rationale for subspecies ID.

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