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Foragers use a variety of anti-predator behaviours to increase their safety from predators. While foraging, animals should alter usage within or between sites to balance the benefits of feeding with the costs of predation. I tested how the distribution of food abundance and predation danger interacts to explain spatial usage (i.e. distance from shore) by migratory western sandpipers ( Calidris mauri) at Boundary Bay, British Columbia, Canada, during northward and southward migrations. At Boundary Bay there are opposing spatial gradients in the distribution of food abundance and safety from predators. Predation danger for sandpipers is high near the shoreline where there is approach cover for falcons and decreases with distance from shore. Food abundance for sandpipers declines as distance from the shoreline increases. Food and danger attributes at Boundary Bay also differ temporally, such that food abundance is higher during southward migration, and predation danger is higher during northward migration. The spatial usage by western sandpipers balances the tradeoff between the opposing spatial gradients in food and safety. For both migratory periods spatial usage of the mudflat by sandpipers is highest at distances from the shoreline where food abundance and predation danger are intermediate. During the northward migration sandpiper usage is highest between 150 and 500 m from the shoreline, and during the southward migration sandpiper usage is highest between 100 and 600 m from the shoreline. Despite temporal differences in food and danger attributes, spatial usage of the site by sandpipers does not differ between migratory periods. Understanding how the distribution of food abundance and predation danger interact to affect the within site usage by shorebirds has important implications for assessments of site quality. [ABSTRACT FROM AUTHOR]

We examined winter habitat preferences of individual dunlin (Calidris alpina) in the Fraser River delta, British Columbia, Canada, adjacent to agricultural land near an area of dense and increasing human settlement. We used radiotelemetry and compositional analysis to quantify and describe dunlin habitat selection at 2 scales (regional and local) throughout the 24-hr day and daily tidal cycles. We tested for differences between sex and age classes, and among birds captured at different sites. Patterns of habitat preference differed between sexes and among dunlin from different sites in the delta, but we detected no difference between age classes. We ranked habitat types in order of dunlin preference and tested for significant differences among habitat ranks. Dunlin showed a significant preference for tidally influenced marine habitats at both scales and throughout the study area. However, most individuals (>80%) also used terrestrial habitats, usually during high tide and primarily at night. The role of terrestrial habitats in the ecology of Fraser River delta dunlin previously had been underestimated because these habitats are used far more at night than during the day. Regionally, soil-based agricultural crops ranked above other terrestrial habitats, and pasture was the only terrestrial habitat that was ranked highly and preferred at both scales. Pasture vegetation tends to be short, and pasture fields in the Fraser River delta are fertilized heavily and naturally with cattle manure. We recommend that managers promote the maintenance of a mosaic of soil-based agricultural crops--with a particular emphasis on naturally fertilized pastures--for dunlin and other shorebirds wintering in the Fraser delta. Terrestrial habitat fragmentation also should be kept to a minimum, as dunlin preferred large fields, likely in response to predation risk. [ABSTRACT FROM AUTHOR]