We would all like to be friends to the birds and enjoy their company. However when a barn swallow builds a nest on your porch you will be tested to the limit by the resultant mess of mud and poop.
We recently had a wonderful nest of four baby barn swallows on our front porch and were debating how to cope with the mess. I solved the poop problem by placing a rectangle of brown paper immediately under the nest with some rocks holding it in place. It turns out that the adults rarely defecate on the porch but the little ones put their rear ends up over the edge of the nest and let fly. Since the mess falls directly down onto the paper below the problem is solved.
When the babies are quite young the adults remove fecal material from the nest. But when older the baby birds themselves defecate over the edge of the nest. This is actually quite a neat evolutionary solution to the issue of how to keep the nest clean. Birds that fouled their own nest must have had more diseases and were selected against. So babies are hatched with the knowledge of how to go to the potty over the side of the nest!
The diaper can be removed and recycled into fertilizer in the garden.