Migratory Distance and Predation Risk as Drivers of Behavioral Tradeoffs Made by Passerines During Fall Stopover

Abstract

Migratory must balance the energetic requirements of large-scale movement with the predation risk inherent in unfamiliar habitats. Optimality theory predicts that species migrating different distances prioritize time and energy differently, and the tradeoff shapes their response to predation risk. We conducted two investigations along the coast of Maine to determine the behavioral and population-scale responses of migrating birds to predation risk as a function of their migration distance. First, we conducted an aviary experiment in 2012 and 2013 at four sites to determine if migratory distance predicts time versus energy tradeoffs and the activity of individual birds under increased predation risk. To isolate the effect of migration distance, we compared the behavior of two pairs of species, each with a longer and shorter distance migrant: yellow-rumped (Setophaga coronata) versus blackpoll (Setophaga striata) warblers and blue-headed (Vireo solitarius) versus red-eyed (Vireo olivaceus) vireos. Individuals were captured at placed into a 5m x 3m x 3m field aviary. We recorded activity (perch changes) before and after exposure to either a control (American Goldfinch Spinus tristus) or predator (Merlin Falco columbarius) call. Second, we used banding records from 2007-2013 for our four focal species to test if behavioral tradeoffs explained size- corrected mass across nine banding sites in Maine that varied in raptor abundance. Migrating raptors were recorded during banding station operation at each site and standardized by total site banding hours. Contrary to the predictions of Optimal Migration Theory, there were no differences in bird behavior by migration distance prior to predator exposure. Further, longer distance migrants were less active under increased risk, while shorter distance migrants were more active. At the population scale, the masses of all four species decreased as predator frequency increased, but mass losses were greater for longer distance migrants. These results aligned with the individual responses we observed in the field aviaries. We hypothesize that the difference in risk aversion by migration distance may be attributed to differential life expectancy between longer and shorter distance migrants and the relative costs that risk aversion creates for migratory refueling and timing. In passerines, longevity is positively correlated with migration distance. Longer distance migrants, therefore, may be more risk averse to maximize survival to future reproductive opportunities, while paying short-term costs to both condition and the carryover costs to the subsequent breeding season. The behavior of shorter distance migrants, however, reflects investment in current condition, which may maximize positive carryover effects for the subsequent breeding opportunity at a cost to survival