I am so surprised there isn't a birdwatching topic (or if there is I have missed it)! I'll try to get a photo but it's exactly like a tree sparrow in size and shape but completely brown and very speckled across the breast and head, a bit like a wren's wing. Looks and behaves definitely like an adult. No idea what it is! It's been coming to my bird feeder the last week and I am a bit baffled. Any ideas