Identifying based on location

[upupa-epops]

I find this article both entertaining and informative on the subject of identifying by location: ICYMI: Indeterminacy « ABA Blog

Let’s proceed. You’re still in California, but now you’re looking at the bird depicted below. Looks pretty good for Iceland Gull. In fact, I’d say it looks great for Iceland Gull. Now, as many birders know, Iceland Gulls are variable. Let’s say “only” 99% of Iceland Gulls are as pale overall as this one, but 1% are darker. Fine. This bird would appear to be among the 99% of Iceland Gulls that are, for want of a better word, “normal.”
…
Enter Thayer’s Gull. Thayer’s Gulls are variable, too—notoriously so. But let’s be reasonable. Let’s say fully 98% of Thayer’s Gulls are darker than the bird depicted here.
…

Anyhow, the case is looking awfully strong for Iceland Gull, you would think. We can’t be completely certain, of course, but I think most of us would lean pretty heavily toward Iceland. We won’t say it’s “definitely” or “positively” an Iceland Gull, but let’s say it’s “likely” or “probably” that species. I mean, 99% of Iceland Gulls look this way, and 98% of Thayer’s Gulls do not. Let’s split the difference, and say this bird has a 98.5% chance of being an Iceland Gull.

Not so fast.

Iceland Gulls are much rarer in California than Thayer’s Gulls. I’m going to say that for every Iceland Gull in California, there are about a thousand Thayer’s Gulls. I think a lot of folks would say I’m being pretty generous in that assessment. Maybe it’s more like five thousand to one. But let’s stick with one in a thousand.

I’ve now given you the three pieces of information that are necessary for determining the probability that this bird is an Iceland Gull. … The answer, surprisingly, is 4.7%. (For the mathematically inclined birder, I work out the numbers in a footnote at the end of this post.) That’s it. That’s all. There’s only a 4.7% probability that the bird is an Iceland Gull. Statisticians generally say that probabilities below 5% tell you you’re looking at something else. Statisticians call that something else an “alternative hypothesis.” We birders call that something else a Thayer’s Gull. This bird is a Thayer’s Gull.

Unless you’re in Pennsylvania.

In Pennsylvania, Iceland Gulls outnumber Thayer’s Gulls by at least ten to one. And that’s being quite generous. If you do the math for Pennsylvania, there’s a 99.8% chance the same bird is an Iceland Gull.

Thankfully, Thayer’s Gull and Iceland Gull have since been lumped. However we still have the three subspecies (and apparently a subspecies hybrid) that people can try to force individuals into, despite L. g. kumlieni probably just being a hybrid swarm between the other two subspecies.

There are lots of cases where assumptions like this need to be made. American/European Herring Gulls, Eastern/Western Wood-Pewees (and now Eastern/Western Warbling-Vireos), etc. Gulls are particularly frustrating since they’re so capable of vagrancy. When I see crows I assume they’re American Crow most of the time, but in certain places in my area I have less confidence because there are Fish Crows around. Fish Crow could occur away from those places but the chances go down dramatically. Generally outside of areas where a second species is remotely possible, you don’t suspect it unless you see a particularly extreme phenotype of the unusual species.

This starts to become an issue when the range limits or intergrade zones are unknown.

@upupa-epops

Toxomerus geminatus and Toxomerus occidentalis look pretty much identical, but their range is well know and on opposite sides of the continent so I feel comfortable identifying them despite that. On the other hand I’ve been researching the millipede genus Brachyiulus and … only one species (pusillus) is documented for eastern Canada, but another species (lusitanus) has been recorded in North Carolina, California, and maybe Ohio. So is it safe to call mine pusillus or should I be careful because Ohio isn’t too far away?

If the observation is within reasonable travel distance of another species, then I would consider that in the probability. A European Herring Gull is marginally more likely on the coast than inland North America. A previously undocumented introduced millipede species is more likely in an urban area with lots of potted plant imports, adjacent to another state which already has that species, than it is in a rural area several states away.