Allison,

I read this paragraph with some interest and a question:

However, the Cornell page shows a Costa's female in Baja California that
clearly has a dark throat patch and green-tinged sides, so maybe females
can't be identified by coloration alone. It's entirely possible that they
are indistinguishable in appearance.

I am no expert on hummingbird ID, BUT...
As a U Chicago trained mathematician and a working UW physicist, I can
safely say that I have never found a physics or math text to be error free,
even later editions. How do we know that the picture at Cornell of a "Baja
female Costa's" is not actually a photo of an Anna's? Did someone get
genetic material from this bird? Was some other ID marker seen? Or as you
have said, they may be indistinguishable? But which is it? There must
surely be errors that creep into bird ID books and onto the Cornell
website. Once an error gets onto a site (or other resource) generally
considered "reliable," that error tends to propagate and be repeated as
"true lore."

Doug Will
UW & Lake Forest park
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