Does anyone know this bird?

Sorry about the quality of the picture. It was taken at high mag. without a tripod LOL. I lightened the picture a little so the dark tail can be seen, and also the line from its eye to beak.
These little (I would guess 4-5 inches sitting, and a wingspan maybe 6-8 inches) birds flit around, hover, and make quick chirping noises. Their bellies are yellow. This is the 1st year I have seen or heard them here. I did a seach for yellow bellied (and buff belly) bird in Colorado, but nothing that I got to looked like these little guys. Any help would be appreciated.

We have them here too...I am pretty sure it is a Western Kingbird.

http://www.enature.com/flashcard/show_flash_card.asp?recordNumber=BD0290

I am an avid bird watcher and belong to the Audubon Society and I looked it up in the Field Guide to North American Birds and it looks like a Phiadelphia Vireo. I think I have seen them too. They just fly through from Guatimala to Canada for the summer.Philly does not fit but a man from there named it. CU BEV

Thank you so much for the replies! The size more-so fits the Vireo, but the bill is completely black. I think it is probably the western kingbird, as they have been here all summer.
I really appreciate you both.

Shelly...here is a couple of sites to help you identify birds in your area.

Also I added The Philadelphia Verio so you could see the difference in the size of the bills. Also the Verio has white bars above the eyes and the bill is quite thinner and narrow compared to the Kingbird.

http://www.mbr-pwrc.usgs.gov/Infocenter/i6260id.html

Here is another picture of the Western Kingbird...and I think this picture comes very close to the picture of your bird.

http://www.mbr-pwrc.usgs.gov/Infocenter/i4470id.html

This site is a great one. I use it much in looking up birds.

Shelly,I would tend to agree with BingsBell on the Western Kingbird as being the bird you saw,I believe the Philadelphia Vereo would be much less likely as they pretty much summer up in Canada.Not that it positively isn't a vereo.It could happen.I know we have the western kingbird here in Nebraska also and it looks like a match.They sound like you described too.I hope you find out for sure either way.Keep us posted as I am most curious!. Cedar

Shelley -- it's a kingbird. They were very numerous in my area starting around APR 25 this year. More than I have ever seen before in nesting pairs. The vireos are much smaller, and harder to see, and only around for a very short time during migration.

Thank you all so much!

I was gong to say it looked like one of the flycatchers... but you already got some nice solid replies. The coloration is completely different from our Eastern kingbird, but the posture is very similar :)