Thanks for the link! I listened to the bird calls and it seems to be a chirping sparrow. At the bottom of that link it says "adult breeding" and "adult non-breeding" Is that indicating what they look like at different times of the year, when it is breeding season? If so I think I have been seeing a mixture of breeding and non which has made it difficult to identify them w/o knowing that there was a difference.
Yes, that's what the labels are showing. A lot of birds already have breeding colors now, but different species have different timing. It does make it tricky when a bird you know in one season looks rather different in the next! Feel free to post more pictures if you're not sure about some at the feeder now.