Foraging opportunity: a method of monitoring shorebird migration and overwintering sites in a changing environment

Roberts Bank within the Fraser River estuary, BC contains important migratory stopover and overwintering habitat for shorebirds such as the western sandpiper (Calidris mauri) and the Pacific dunlin (Calidris alpina pacifica). Shorebirds are especially abundant during northward migration, with single-day counts numbering into the hundreds of thousands of birds. Previous research and ecological theory have demonstrated that site usage by shorebirds is influenced by numerous factors, including prey availability and predation risk. We developed a concept termed “foraging opportunity” that quantifies shorebird food availability (biofilm, meiofauna, and macrofauna) in relation to predation danger from hunting falcons. Foraging opportunity was determined across Roberts Bank during northward migration and evaluated against shorebird usage for the same period. Model results agreed with prior research and foraging theories, demonstrating good alignment between prey resources and shorebird usage in safer foraging areas and a shift in usage into areas with less prey, but safer foraging conditions, when high prey abundances were located in more dangerous conditions close to shore. As shorebird migration and overwintering sites are potentially affected by a changing environment, including climate change and anthropogenic effects, we suggest foraging opportunity techniques as a method of understanding and monitoring site quality and shorebird distribution patterns in a changing world