Screening might be too brittle or too stiff...maybe a little bit of cheesecloth? It won't keep the bees from sucking the nectar off the cloth, but it would at least keep them out of the feeder...
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."