Some background on subspecies
Subspecies are a common focus of debate among birders and ornithologists. The subspecies concept exists as a way to record and classify variation below the species level. The problem is that once we venture below the species level there is no consistent set of rules governing the naming of subspecies. And so, the current collection of named subspecies includes some that differ only slightly and inconsistently in plumage color (like Southeastern and Northeastern American Robins), and other populations that are distinctive in plumage, voice, structure,and behavior (such as Eastern and Western Willets). By calling both “subspecies” we give them the same rank, which is misleading and undermines the purpose of naming them in the first place.
Many attempts have been made to set quantitative and objective rules for naming subspecies, but these always fail when they run up against real-world variation in birds. The variation in subspecies is itself too variable and too ambiguous to fit any simple set of rules.
Scientifically, we should avoid the categorization imposed by names and describe variation in paragraphs of text and in graphics showing the patterns of variation across regions. By naming a few subspecies we often obscure clines and other subtle and complex regional variations.
Realistically, for the sake of communication, we need names and the categorization they create. Birders want a simple way to record information about the variation that they see in the field – a box to check on the day’s list – rather than having to list all the juncos as “Dark-eyed” and then describe the observed forms in the notes. It is important to remember, though, that as a rule the identification of subspecies is subjective and uncertain and often doesn’t fit neatly into our categories.
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.
The subspecies listed here are by no means the deepest level that birders should try to reach. I’m considering the question from a continental scale. You can consider your own local patch to be the “island”, and ask essentially the same question. With a more limited list of likely possibilities you will be able to identify more subtle variations. Phenology will also be a great aid. For example, by behavior you will often be able to identify a “local breeder” as distinct from a “passage migrant”, even if the birds look and sound identical.
The bottom line is that every name simultaneously elucidates and obscures variation, and we should always be aware that there is hidden complexity and subtlety within every subspecies.
Work on the list is ongoing, and if you want to read more “behind-the-scenes” stuff there is a discussion at Birdforum here.
- I have arbitrarily chosen some English names for subspecies, usually referring to the region in which they breed. Many of these could be improved, or there may be more-widely-used English names that I’ve ignored. Please let me know.
- The list is often vague, with the phrase “more study needed” repeated over and over. There is still a great deal to learn about North American birds, and I know that many of these questions are being investigated now and the list will need frequent revision. All comments and updates are welcome.
- Some species are listed with an annotation but no subspecies names. In most of these cases I think there might be identifiable subspecies, but don’t feel that anything is gained by using the names until more is learned about variation and identification in that species.