Our harvest baskets are hung from the barn beams. I wanted one particular basket the other day and a bird flew out and scolded me proper! I took the basket down, saw a nest (no eggs) in it, and promptly hung it back up. I've seen the bird several times since, and I'm pretty sure it's a Bewick's Wren. Since the light in the barn is a bit dim I can't be absolutely sure. We have good lights in the coops, but none in the aisleway. I may have to bring another harvest basket to the barn since it seems the bird basket will be out of commission for a while (and in need of a good scrubbing when vacated.)
Your harvest basket obviously looked like a great place to build a home for its young. Hopefully it will go on to have a productive breeding season.