"As many of you know, it's virtually impossible to distinguish a female Allen's from a rufous except in the hand.
This bird was smaller (wing chord in particular) than a female rufous, and the R2 feather
(not the central feather, but the next one out) lacked the distinct emargination typical of adult female rufous.
The clincher was the extremely narrow outermost tail feather, R5, which was 2.41mm wide.
I've just gotten off the phone with hummingbird expert Bob Sargent in Alabama, who looked at the photos
and measurements and concurs that she's an Allen's."

--Scott Weidensaul, 12/12/09--

Note: The tail feathers were taken under federal permit to serve as permanent voucher specimens.
The hummer will regrow them in a matter of weeks - it doesn't hurt the bird at all.