Passerine migrants respond to variation in predation risk during stopover

During travel, migrants may encounter unfamiliar habitats and predators and visit sites for which they lack information on predation risk. Temporary stops during migration permit only limited opportunities to gather information about risk, and other priorities, such as the need to forage, may restrict these opportunities. Under these conditions, the extent to which migrants respond to variations in predation risk is unclear. We studied small passerine birds during migratory stopover to determine whether their behaviour was related to variation in risk of capture by migrating birds of prey. We used two approaches, observational and experimental. Observations revealed that blue-grey gnatcatchers, Polioptila caerulea, and American redstarts, Setophaga ruticilla, moved deeper into oak shrubs as the number of sharp-shinned hawks, Accipiter striatus, at the site increased. Furthermore, blue-grey gnatcatchers moved at slower rates as counts of hawks increased. The experiment revealed that blue-grey gnatcatchers had lower rates of both movement and foraging after exposure to a gliding model hawk. These results provide evidence that predator avoidance remains a priority during migration and that migrants are able to assess risk to some extent during temporary stopover. In addition, the results suggest that predator avoidance behaviour may limit foraging opportunities during stopover by restricting habitat use.