I believe this little cutie is from the Warbler family, but I'm only basing that on his/her beak shape and body size. I have searched online and in my books, but can't even find a bird that looks similar.
The barring on the main body is what is throwing me off, that and the one bit of yellow under each wing.
It's also a bold little bird and stays up late. It will be at the feeder until it's nearly dark and all the other birds have gone to bed. It will perch 3 feet away when I am putting feed out in the morning.
It is not a bird that spends its entire year here (in Central Texas), either. It showed up about a month ago, possibly on its way back north?