It is fun to hear them isn't it?! There is a male that visits my yard who is very vocal. I found a site that has recordings of many different hummers. Here is the address for the anna's hummingbird. http://identify.whatbird.com/obj/167/_/Annas_Hummingbird.aspx
You should see a "Listen to Call" beside the picture. Under the "Range Map" there is a list of other hummingbirds.

So, they do sing! I thought I was hearing voices :-).
The "Anna's" site is great - I enjoyed listening to many birds' calls. Thanks Sherca!

Thanks I think I have Anna's They do a lot of twittering besides trying to dive bomb the other hummers.