Sure looks like a cowbird egg to me.

Thanks guys, I followed your advice and removed the egg. I made a home-made hole reducer out of a piece of cardboard and taped it inside the box to the front of the box. It's been taped to the hole from all angles. Hoping it will hold up. It is a 1 1/4" or as close as I could get eyeballing it. The chickadees accepted it after a few minutes. Cardboard was the only thing I could think of that is flexible yet sturdy since I have gilbertson boxes. Crossing my fingers for no cowbird attacks. Never had a cowbird egg in 13 years of backyard birding, but now my 1st year at my new house and 1st nest of chickadees ever, I get one.

Interesting. I knew Brown headed cowbirds could not be raised by Goldfinches because they feed their young seeds, but I didn't know House Finches did also. I looked it up at Cornell's lab of ornithology, and they confirm what you have read. I guess that means you don't have to worry about the young House Finches. I've also read about retaliation from the Cowbirds, so I guess just leaving it there should protect the baby House Finches best.

It's illegal to move native bird nests, which probably includes incomplete nests.. According to Wikipedia : "Nests are made in cavities, including openings in buildings, hanging plants, and other cup-shaped outdoor decorations. " I honestly don't think they would mind that much, but it's best to keep your activity to a minimum once the eggs are layed/hatch.However, if it is really a bother, you should take it down..The female can built a new nest in 2 days anyway.. If you do let them nest,take it down once they fledge..This way, they don't reuse the nest..