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How to keep birds from nesting
Use porcupine wire to deter birds from landing on rafters.
Porcupine wire creates an uneven landing place, which makes any birds nesting there unlikely. Lay out a length of porcupine wire over any rafters where you don't want birds to nest to keep them away.
Porcupine wires have heavy prongs that stick out in every direction, and they can be purchased online or at most hardware stores. Lay out bird netting over large areas where you don't want birds to nest.