Sure looks like a cowbird egg to me.

That smaller egg is definitely a Cowbird egg. I got in a HEAP of trouble a few weeks back when I was trying to get some advice on this subject (see the reg. birding forum under "Cowbird Egg in Finch Nest - probably on about pg. 3 by now, for the fracas...), so tread very carefully. I - how shall I say - "took care of" the egg in question. Only you can choose what to do with your interloper egg, but be ready to get flamed by most of the GW birding world for your "illegal, unethical, criminal, (et al)" behavior if you mention it here.

Awesome! Its really a surprise. It feels great when I see birds lay eggs in the nest. I use to live in the farm and took care of all the different animals there.

So far I've only seen cowbird babies here with Red-winged Blackbirds and Baltimore Orioles. The size is similar and there seem to be mixed families so the cowbirds don't appear to have displaced the original babies. The red-wings and cowbirds feed together amicably and the orioles brought both cowbird and oriole fledglings to the jam feeders (although the cowbirds didn't eat the jam). None of the cardinals, blue jays, or grackles have cowbirds associated with them, so they probably rejected them. No crows either, although they're fledging now and the adult cowbirds are probably long gone. I can't imagine a crow putting up with a cowbird chick, anyway. I wonder if the cowbird fledglings identify with the birds that raised them, so that a cowbird female raised by a red-wing would be more likely to lay an egg in a red-wing nest. That should give a higher rate of success and maybe skew the egg-laying process toward species like red-wings and orioles. Claire