You can use a frame of small sticks and staple blueberry netting to create a 'cage' to go over the top of the lettuce, it doesn't have to be very large, just tall enough so that the birds can't get their beaks through. You can find blueberry netting usually at any home improvement store, garden center or hardware store.
Screens like this Hometalk member came up with to deter squirrels would deter birds as well:
We put a sprinkler on the seeds regularly until they are about half an inch tall. The birds lose interest about then.