Screen Out Birds
Make a wooden frame that's just slightly larger than your lettuce patch and staple window screening to it. Next, pound stakes into the soil at the corners of the lettuce patch to prop your screened frame just a few inches above the soil, and lay the frame on top of them. Of course, if you have or can find some old, ready-made window screens of the appropriate size, all the better.
Your homemade shield will let sun and water in but help keep most of the thieving birds -- as well as some other hungry pests -- away from the seeds and the tasty lettuce leaves.