Sibley’s Guide To Birds remarks that the Black-Chinned has a grayer crown and pumps it’s tail frequently while hovering.
I have a female hummingbird that doesn’t look or act quite the same as others at the feeder. The crown is nearly solid gray and it does pump. I don’t see a length difference in the bill but in my defense it’s head is usually buried in the fluid tube.
Dare I to think it might be a Black-Chinned ?