A couple of comments re: the recent hummer thread and a question.
In 43 years here in Carver, in the last 10 or so many many Anna's and Rufous, I've seen
1. A hybrid Anna's Rufous. Looked mostly like Rufous but had a red forehead/crown. I was excited, got my camera and waited for it to return to my feeder. It didn't. This hybrid was apparently first recorded by John Gould with a colored picture in 1861 in his magnificent book on hummingbirds. Not know as a hybrid at the time he called it Floresci's Hummingbird. See footnote.
2. Only once have I seen a green-backed Rufous. Quite extensive amount of green but undoubtedly a young or aberrant Rufous. Or perhaps a far flung hybrid. Once again, too furtive for a photo.
Now the question. In terms of talk about differentiation by mating display or by call (or even by emargination of the tail feathers), you can undoubtedly separate Allen's and Rufous. Except for one consideration. What do their hybrids offer in this respect?