Joan, on p. 46 of *Wild Bird Guides: RubyThroated Hummingbird* by Robert Sargent, there's a picture of a female with a patch of white on the top of her head. Not a cap exactly, but a noticeable white patch like a little yarmulka.

My guess is it's just a normal variation, even tho not too common.

You're lucky to have such a distinctive marking so you can esaily recognize her :)

Thanks, Mehitabel! A yarmulke more aptly describes her "cap." I will have to check out the suggested photo. Appreciate the tip....we so enjoy her visits to our feeder!