We've several likely suspects: Couches or Western Kingbirds, perhaps a Great Crested Flycatcher.
Check out the photos at
http://birdmecca.posterous.com/oh-those-hard-to-id-yellow-fellas?ref=nf
Check out the photos at
http://birdmecca.posterous.com/oh-those-hard-to-id-yellow-fellas?ref=nf