Predation risk and the predator avoidance behavior of migrant birds during stopover
During migratory stopover, birds forage to replenish energy stores needed for long-distance flight. However, foraging exposes them to predation by migrating birds of prey. The prey species can mitigate this threat behaviorally, but at the cost of reduced foraging efficiency. Thus, much behavior during stopover can be viewed as the outcome of this safety/foraging trade-off. This study had four objectives: (1) describe predation risk during stopover, (2) determine how migrants respond to variable risk, (3) determine what factors affect predator avoidance during stopover, and (4) determine whether vocalizations facilitate flocking, a common antipredator tactic. To achieve these objectives, I combined both field experiments and observations at a site on the Alabama coast. The passage of bird-eating raptors (Order Falconiformes) was variable and unpredictable among days and years, yet a clear pattern emerged when data were examined at a 5-day time scale and averaged across years. A predation risk index based on the ratio of indices of predator and prey abundance also varied greatly among days and years, but formed no clear pattern when viewed at the longer time scale. These results suggest that birds migrating during the same period as the raptors should be selected to respond behaviorally to short-term variation in risk. Foraging observations revealed that the behavior of two potential prey species was indeed related to the abundance of bird-eating raptors. Both Blue-gray Gnatcatchers ( Polioptila caerulea ) and American Redstarts (Ruticilla setophaga ) moved at progressively slower rates and foraged deeper within shrubs as numbers of predators increased. I obtained similar results by experimentally exposing Blue-gray Gnatcatchers to a flying hawk model. Experimental evidence revealed that fat can affect the predator avoidance behavior of Gray Catbirds (Dumetella carolinensis ). After exposure to a model hawk, fat individuals remained motionless longer than lean individuals. Gray Catbird alarm calls also affected behavior. Those exposed to a broadcast of the call remained within a patch of cover longer than those exposed to a control recording. Blue-gray Gnatcatchers commonly participated in flocks at the site. Furthermore, flocking individuals called differently than solitary individuals suggesting that vocalizations provide flocking status information. Experimentation revealed that Blue-gray Gnatcatchers are attracted to conspecific calls. Thus, this species apparently uses vocalizations to find or recruit conspecifics upon arrival.