Answer: As you have discovered, pigeons can be pretty smart birds. Unless the plastic owls are moved every few days, the birds will figure out they're not much of a threat and will continue roosting on your roof. You might try using the bird-scare tape that's been successful in keeping birds out of strawberry beds. It's about the size and texture of tape found in old eight-track or cassette tape reels. When strung from one post to another and pulled tight, it gives off a humming sound that only birds can hear. They seem to avoid the area, especially when there's a slight breeze blowing. One other possibility is to string nylon fishing line criss-cross fashion, about 3-4 inches above the top of the tile on your roof. The line is almost invisible and the birds may avoid the area if they think they might get tangled up in it.
I can't vouch for the effectiveness, but there's a repellant spray on the market called 4-The Birds. Information is available at the following website:
Hope the above information helps!
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