Investigation of Breeding Peregrine Falcons on Virginia Bridges

About the report:

Following the extirpation of the peregrine falcon (Falco peregrinus) population in Virginia by the early 1960s and an aggressive restoration program during the 1970s and 1980s, the population has undergone a slow but steady recovery to more than 30 breeding pairs. Bridges have played a significant role in this recovery, consistently supporting more than 30% of the known population. Due to regulatory restrictions, this role has increased operational costs and caused concerns for bridge management and maintenance. One of the ongoing challenges faced by the Virginia Department of Transportation (VDOT) is the uncertainty and associated financial risk stemming from not knowing the occupancy status of many bridges. The objectives of this study were (1) to determine occupancy for bridges in the Coastal Plain, (2) to test a rapid survey protocol for determining occupancy, (3) to assess bridge characteristics that attract falcons to bridges, and (4) to conduct a retrospective assessment of current peregrine management techniques used on bridges.

The authors conducted 166 surveys of bridges (n = 83) in coastal Virginia using a call-broadcast protocol. Eleven (13.3%) bridges were occupied by falcons, including 10 pairs that produced 11 young. Broadcast calls were extremely effective in eliciting a response from falcons with nearly 60% and 100% of falcons responding within five and 30 seconds of call initiation, respectively. Occupied bridges were not a random subset of those surveyed but supported more potential nest sites, were longer and higher, and were embedded within landscapes with more foraging habitat compared to unoccupied bridges. Lift and draw bridges were particularly attractive, with 60% of those available supporting pairs. The current practice of installing nest boxes or trays has resulted in higher breeding success and reproductive output.

Findings from this study have implications for reducing uncertainty in peregrine falcon management on bridges. Call-broadcast surveys were effective and should be used to expand the set of bridges monitored annually to improve the planning and scheduling of maintenance projects. Current peregrine management techniques improve breeding performance but may also reduce maintenance conflicts and should be continued. The bridges identified in this study as occupied by falcons or that have a high potential to be occupied in the near future will be communicated to the appropriate VDOT staff.

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