Cliff swallows (Petrochelidon pyrrhonota) are migratory birds that breed in colonies and frequently nest on highway structures. Protected by the Migratory Bird Treaty Act of 1918, swallows in their active nests cannot be harmed by nesting-control methods. This causes problems and delays in maintenance of structures by divisions of many departments of transportation. We evaluated 2 aversion strategies, bioacoustic deterrents and surface modifications, for their effect on cliff swallow nesting behavior. The bioacoustic deterrents consisted of sonic devices that broadcast 8 unique recordings of alarm and distress calls of cliff swallows. We made surface modifications, mounting high-density polyethylene sheeting on the vertical surfaces at typical bridge-nesting locations. We used 28 bridges in the Sacramento Valley of California to test the aversion strategies. Both the broadcast calls and polyethylene sheeting treatments significantly reduced the number of nests built at a site, but neither treatment nor the combination of treatments completely stopped nesting, as would be required by transportation departments.

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