Close encounters: Behavioral responses of migrating songbirds to the perceived risk of predation

Migrating birds face competing pressures to travel as quickly and efficiently as possible while minimizing the risk of predation en route. Despite the potential importance, antipredator behaviors in migrating songbirds have been little studied relative to time and energy tradeoffs, even as humans have introduced novel predators including free-roaming domestic cats (Felis catus) across the globe. Birds confronted with predators during stopover can employ antipredator behaviors to reduce their immediate mortality risk, but doing so may slow refueling or impose other costs that influence migration. We captured migrating Gray Catbirds (Dumetella carolinensis) and exposed them to either a live cat, model hawk, or non-predator control in an aviary experiment to assess their behavioral responses. Birds moved lower after exposure to a model hawk, but showed no significant behavioral changes in response to a cat. After release, we monitored activity levels of a subset of tagged individuals via the Motus Wildlife Telemetry System. Post-release activity level did not differ between treatment and control groups, indicating that brief predator exposure did not exert a persistent effect on this behavior. The lack of detectable responses to the cat may reflect birds navigating the tradeoff between antipredator behaviors, which are costly during migration, and reduction of predation risk. However, it could also indicate naïveté of young birds to this introduced predator, which may increase vulnerability to predation during migration. We encourage further investigation of the influence of prolonged or repeated exposure to domestic cats on songbird behavior and physiology, and ultimately migration success.

Methods
All pre- and post-test behavioral observations were recorded by C.E. Nemes using Gray Catbirds captured during fall migration. Methods for data collection and processing are described in the associated paper.

For the activity analysis, a subset of catbirds were nano-tagged by C.E. Nemes. Detections of tagged individuals at local receivers were downloaded from the Motus Wildlife Telemetry System website (www.motus.org) using the Motus R package and de-trended signal strength values were calculated as described in the paper. Motus detection data are freely accessible but must be cited, which is not compatible with Dryad's CC0 policy. Hence, we have archived detection data for the activity analysis at Zenodo.