No, it’s not the fight of the decade—it’s one of the thorniest bird ID problems in North America. Right now little orange and green hummingbirds are sweeping across the continent on their way south. Most are traveling through the Pacific and Intermountain flyways and will end up in Mexico, but a significant minority (hundreds) will stray east of the Rockies to delight and confuse migration watchers and winter hummingbird aficionados.
An unambiguous adult male Rufous. If the back is less than half green (including entirely rufous), you can safely call it a Rufous without seeing the tail. (Note the green crown, which is normal, and the green gorget, which is an artifact of the angle.)
One of the most commonly repeated myths about hummingbird identification is that an orange hummingbird with a green back is an Allen’s. I was told this by a local birder on my first visit to southeastern Arizona in 1978, and on her authority I put Allen’s Hummingbird on my life list based on the little orange and green female-plumaged birds swarming around her feeders. There it remained until 1988, when I moved here and began to acquaint myself with the true depths of the problem.
Once I realized that in both Rufous and Allen’s all females and juvenile males have green backs, I scrubbed Allen’s from my life list. Over the next few years I learned through banding experience that a small percentage of adult male Rufous have enough green on their backs to be easily confused with Allen’s.
Yes, I know you don’t want to hear this, but take your fingers out of your ears and look at the photo at right (you may click the image to embiggen).
Notice the notched tip of R2 (the next-to-center tail feather)? Diagnostic for Rufous. Sorry.
The only safe, accurate way to distinguish between Rufous and Allen’s in any and every plumage is by the shapes of the tail feathers. You can see these when the birds fan their tails in combat or preen them. In Allen’s, all of the tail feathers are narrower than in Rufous, most noticeably the outer three. In Rufous, R2 has that distinctive notched tip in adult males, expressed as a “pinched” tip in most (but not all) adult females and juvenile males. To clarify, here are silhouettes of the adult male tails:
And here are juvenile males (note the green backs):
You can see how extremely subtle the differences are in juvenile males (adult females are similar)—not something you’re usually going to see in the field. Juvenile females are the most “generic” and can be impossible to identify even in hand. This is why it’s so important to determine the age and sex of the more difficult hummingbirds before you try to assign them to species. If it’s a female or juvenile male Rufous or Allen’s, best to fuggedaboudit unless you can get photos of the fanned tail.
It’s very common for orange-and-green hummingbirds observed east of the Rockies to be called Rufous in the absence of any documentation, based simply on expectation. Sometimes “probable Rufous,” sometimes “Selasphorus species” or “Rufous/Allen’s” (which we’ll get to in a moment), but all too often just “Rufous.” This can give the impression that an identification has been confirmed when it hasn’t, leading to much rarer birds (Allen’s, Broad-tailed, Calliope) being overlooked. A look at the range maps in A Field Guide to Hummingbirds will show you why it pays to know all of the Rufous doppelgangers’ field marks and check them out for yourself (documenting with a camera, where possible).
More conservative birders may go a bit too far the other way, calling any orange-and-green hummingbird “Selasphorus species” even when Broad-tailed (the oft-forgotten member of the genus) is readily ruled out. Once Broad-tailed has been eliminated from consideration, the most accurate label to use is “Rufous/Allen’s.” You’ll find this as an option on eBird and “Allen’s Hummingbird/Rufous Hummingbird” in the Christmas Bird Count historical results. If any orange-and-green hummingbirds occur within your local CBC circle but the compiler doesn’t use “Rufous/Allen’s” on tally sheets or in the final reports, please encourage them to do so.
Addendum 1: I’ve created galleries on Flickr with comments on each photo pointing out the key field marks that identify each bird as Rufous or Allen’s.
Addendum 2: Since this post was published, Calliope Hummingbird has been moved to the genus Selasphorus, making “Selasphorus sp.” even less specific than it used to be.
Addendum 3: Though the Sibley Guide illustrations point out an orange “eyebrow” on the side view of the adult female Allen’s but not on the (virtually identical) adult female Rufous, this is a field mark distinguishing Rufous and Allen’s from the gray-faced Broad-tailed and Calliope, not from each other.
Addendum 4: Clearly Rufous by the widths of all the tail feathers and the distinctive shape of R2, but with green all the way to the uppertail coverts:
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On a historical note, this time of year—Rufous time—always brings to mind the late Ken Kesey and the Merry Pranksters. Orange and green were the heraldic colors of these counterculture court jesters. Toward the end of Ken’s life, one of the networks came to his Pleasant Hill, Oregon home for an interview. Ken took them out onto his property to show them the Pranksters’ original bus, “Further” (also spelled “Furthur”). When they reached the bus, Ken noticed a small shape buzzing around inside: a Rufous Hummingbird. Ken gently corralled the terrified creature and carried it to freedom outside Further’s door. I cried.