I don't think the few drops of dish soap would stop the water from evaporating, but rather was use to reduce the surface tension of the water and prevent some bugs/insects from walking across it.

Much to my surprise we had a hummingbird in our yard last year. So I guess to keep my daughter entertained we'll be feeding it this year

Doug, I look forward to more pictures of your hummingbirds, last years were quiet fantastic.