Relationship Between Stopover Site Choice of Migrating Sandpipers, Their Population Status, and Environmental Stressors

Abstract

Measures of animal behavior can be used in a variety of situations to make inferences about the environment and population status. Work by our research group shows that migratory shorebirds adjust their usage of, and behavior at, stopover sites in response to environmental conditions. Motivated by this, we built an individual-based model of migrating shorebirds moving through a sequence of alternating small and large stopover sites. Birds at larger sites are safer from predators, but we assumed that less food is available than at small sites. In the model, both predation risk and food intake are densitydependent, and the behavior of migrants is controlled by two rules: one that determines whether a bird will depart a stopover site, and one that controls the individual's foraging versus vigilance intensity. The optimal behavior is calculated by maximizing a payoff function that depends on arrival date and arrival energy stores at the final site. We used this model to predict mass gain, foraging intensity, and usage by migrants of small and large sites under various conditions. We examined the effects of a flyway-wide reduction in the amount of food, a flyway-wide increase in predation danger, and the effects of lowering the overall population size. The mass action of many individuals, each optimizing its migration timing and routing, leads to the emergence of distinctive patterns of behavior and site choice under these differing environmental conditions. When food availability is reduced throughout the flyway, our model predicts that foraging intensity increases at every stopover site, thereby forcing birds to accept greater danger to maintain the fitness benefit of a timely arrival to the breeding area. A flyway-wide increase in predation danger results in fewer migrants choosing (and/or migrants staying a shorter time at) small stopover sites, balanced by a higher usage of large sites. These effects contrast with what is observed under true population declines, when the usage of both small and large sites declines.

Keywords:

• bird migration
• genetic algorithm
• behavior
• optimization
• departure rules
• stopover ecology
• shorebirds
• Calidris mauri