Willow Flycatchers say much more than "fitz-bew" and the call
you describe sounds like a typical Willow "zwee-oo" call.

But you don't have to take my word for it...
http://www.appliedbioacoustics/Repertoires/Passeriformes/Tyrannidae/wiflalfl.html

--
Mike Patterson
Astoria, OR
celata at pacifier.com

In Praise of Urtica
http://www.surfbirds.com/blogs/mbalame/archives/2008/06/urtica20080622.html