California Department of Transportation

Methods for Excluding Cliff Swallows from Nesting on Highway Structures

The cliff swallow is a migratory songbird found in most of California. This species typically nests on bridges and other structures. Colonies may contain several hundred nests. Cliff swallows are protected under the federal Migratory Bird Treaty Act of 1918. Thus, it is illegal to remove completed nests during the breeding season without a permit from the U.S. Fish and Wildlife Service. The cliff swallow's protected status and its habit of nesting on bridges can create costly delays in construction and maintenance activities for Caltrans. For Caltrans’ activities to proceed on schedule, netting to exclude the swallows is often installed prior to the birds' arrival.

However, netting is not problem-free and other techniques to deter nesting, merit investigation. In addition, the development of an empirical habitat model for cliff swallows would allow Caltrans to predict swallow presence at specific sites and plan in advance for mitigation.

The objectives of this research are to:

  • Summarize the existing knowledge of cliff swallow ecology, behavior, and control strategies;
  • Conduct field surveys of cliff swallow behavior and habitat at occupied and unoccupied sites to predict the likelihood of occupancy and to identify potential control strategies; and
  • Evaluate the most promising control strategies on selected sites and select the best approach for future implementation.

This research will provide Caltrans with new control techniques for preventing swallow nesting and a model to identify likely colony sites and thus plan appropriate mitigation. The new control techniques will also have application to cliff swallows and barn swallows nesting on buildings and other structures.

Cliff Swallows Nests Under a Highway Structure
Cliff Swallows Nests Under a Highway Structure