This is a sanitation product that is supplementing bird protection. This is a 20 x 15 cm sheet that can be made of wood (like the one we sell), metal or plastic, and it has to be attached below the nest. The diaper keeps the feces, so the surface below will stay clean.
You should remove the accumulated, dry feces in the fall, but if needed you can when the birds are still in there. If you must, you can attach the diaper under an active nest, but not when the offsprings are still in eggs, because when they are out, the parents won't leave the nest.
Artificial swallow nest supplemented by swallow diaper: