Thin wires strung above a beach to keep the birds away. Often made of fishing line. Sometimes called "gull nets" or "gull netting," which is misleading.
Seagulls, in particular, can be a threat to a good beach, especially a small, densely used one in an urban area away from the ocean. Such beaches tend to be found at riverbends or in still bays or coves on lakes: these are sheltered spots where the water doesn't move as fast and has a chance to warm to a comfortable temperature in summer. These qualities also make such spots appealing to germs.
Threatening bacteria, such as E.coli, build up like crazy in warm, still patches of water. They're particularly happy when they have a lot of bird doo on which to feed.
Humans have a habit of throwing aside bits of food when they're done picnicking, and gulls will eat just about anything. Put a lot of beachgoers, a lot of trash, a lot of birds, a lot of bird leavings, and a lot of shallow and warm water together, and you're going to get sick people, especially children.
Gull wiring is virtually invisible to people -- only a few wires have to be strung to keep the birds away, and usually all that's visible are the poles the wires are strung between and a few strands of spider-webby stuff 30 feet in the air that shine a bit when the light's right.
The birds absolutely hate them. They can't see the wires -- which aren't really dangerous, but potentially, you know, embarrassing -- any better than you and I can, but they're trying to fly around up there. Even if they were clever enough to land away from the wires and strut over to the sunbathers to demand scraps, they'd have no good escape route if, for instance, somebody's dog took an interest in them.
It's like magic, like a friendly witch cast some kind of anti-gull hex o'er the land. A nasty, sick-making beach can be opened for public use with the investment of relatively little money. You can achieve much the same effect at a campsite or cottage by running some fishing line around the trees.