The image shows a highly tapered retrix #2 or R2. (You count tail feathers from the inside out. The middle tail feather is R1, then R2, etc.)

Sherri Williamson, author of a A Field Guide to Hummingbirds, gives more information on how to visually separate Rufous and Allen's Hummingbirds on this website, stressing the importance of tail pattern.

"Adult female Rufous showing species-diagnostic "pinched" tip and sex-diagnostic green subterminal band on R2." - On this hummer, pinched R2 - yes, green band - not sure.
"Juvenile males have no green in R2, less white in R3, and (usually) bolder gorget spotting." - Green band - not sure, lots of white on R3, bold gorget spotting.
"Juvenile female with moderate "five-o'clock shadow" and unusually distinct "pinched" tip on R2." - This comment should be interpreted as juvenile females don't typically show a pinched R2. On our bird, R2 is pinched and there's more than a five-o'clock shadow.

Given the shape of R2, and using information and photos of both species shown at her site, it seems to match up to adult female or immature male Rufous Hummingbird. Matthew Cvetas, Illinois Birders Forum.