Longer distance migrant passerines are more predation risk averse on stopover

Grunzel, David; Olsen, Brian

Migratory animals face the difficult task of balancing the energetic requirements of large-scale movement with the predation risk inherent in unfamiliar habitats. Optimality theory suggests prioritizing time or energy along the way will shape migration for different species, particularly in response to variation in predation risk. Migration distance may place further constraints on these strategies. We conducted an aviary experiment at four sites along the coast of Maine to determine if migratory distance predicts time versus energy tradeoffs and, under increased predation risk, activity versus vigilance tradeoffs. We used data from nine banding stations in our region to test if behavioral tradeoffs between migration strategies were evident in size corrected masses at sites with high and low raptor abundance during migration. Neither longer nor shorter distance migrants exhibited behavior in accordance with the predictions of Optimal Migration theory. Longer distance migrants were less active under increased risk, while shorter distance migrants were more active. Losses in mass for longer distance migrants at high risk sites reflect their behavioral tradeoff. These passerine species exhibited two distinct behavioral responses in the face of increased predation risk. Rather than optimizing for time or energy, the difference in risk aversion may be attributed to differential life expectancy between longer and shorter distance migrants.