Feeding and predation danger tradeoffs in stopover site usage by western sandpipers (Calidris mauri)

Migrant birds face a tradeoff at stopover sites between acquiring resources to fuel migration and avoiding predators. This tradeoff is crucial because the rapid acquisition of high-energy fuel for long-distance flight requires feeding in high-resource habitats, feeding intensely, and carrying heavy loads of fat, all of which elevate predation danger. I investigated how migrant western sandpipers (Calidris mauri) trade off food and safety at both the site (within stopover) and landscape (among stopovers) scales. I studied within-site usage by sandpipers at Boundary Bay, located in the Strait of Georgia, in southwestern British Columbia. The mudflat is characterized by a strong feeding-danger gradient, with both food and danger decreasing with distance from the shoreline. I measured dropping densities on transects to evaluate how sandpipers distributed their usage across the mudflat. Dropping densities peaked at intermediate distances from shore, showing that sandpipers maximized neither energy gain (highest close to shore), nor safety (greatest far from shore). The observed pattern is that expected if these factors are traded off against each other. To test this hypothesis experimentally, I manipulating danger by adding obstructive cover to the open mudflat. As predicted, usage was lower on obstruction transects than controls; the difference in usage between control and obstruction transects was greatest near the obstruction; declined with distance from the obstruction; and was greater where food abundance was lower. Western sandpipers were also captured on noose carpets spread across these transects, which revealed that usage was mass-dependent: heavy individuals fed far from shore. A dynamic state variable model incorporating trade off assumptions predicted all of these patterns. I investigated whether similar considerations applied at the landscape scale. Of 17 potential stopover sites for sandpipers in the Strait of Georgia and Puget Sound, only 8 were used on migration. Consideration of both food and safety better predicted stopover site usage than either factor alone. Furthermore, heavy sandpipers predominated at safe stopover sites, while leaner birds used more dangerous sites. This thesis highlights the importance of the interaction between food abundance and danger from predators in studies of stopover site behaviour, usage, and site selection by migrant birds.