Frank,

I’m not totally sure I know what you mean when you say you assigned a keyword to “all Places”… do you mean that all photos that have Places assigned also now have the keyword “Avifauna”?

Regardless, there is a way to remove a single keyword from a batch of photos, by using the keywords buttons bar (menu Window > Show Keyword Controls). Open the button bar editor, and create a button for your keyword. Then do a search through the entire Library for that keyword, so you’re looking at all images with that keyword on them. Finally, select all photos, and shift-click on the keyword button to remove that (and only that) keyword from all photos.

-Joseph @ApertureExpert