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Birds in ceiling

We have a 60's weatherboard house, iron roof. Birds are getting under the roof through the guttering, pretty much all around the house, and more every year. We want to clean out the nests, and then stop them getting in before nesting starts again. Not sure how to stop them. Prefer not to have gutter covered or filled, as gutters arent a problem otherwise. We also need to replace a rotting weatherboard & repair or replace rotting window sill.