Combined measurements of prey availability explain habitat selection in foraging seabirds.

Understanding links between habitat characteristics and foraging efficiency helps predict how environmental changes influence populations of top predators. This study examines whether measurements of prey (clupeids) availability varied over stratification gradients, and determined if any of those measurements coincided with aggregations of foraging seabirds (common guillemot Uria aalge and Manx shearwater Puffinus puffinus) in the Celtic Sea, UK. The probability of encountering foraging seabirds was highest around fronts between mixed and stratified water. Prey were denser and shallower in mixed water, whilst encounters with prey were most frequent in stratified water. Therefore, no single measurement of increased prey availability coincided with the location of fronts. However, when considered in combination, overall prey availability was highest in these areas. These results show that top predators may select foraging habitats by trading-off several measurements of prey availability. By showing that top predators select areas where prey switch between behaviours, these results also identify a mechanism that could explain the wider importance of edge habitats for these taxa. As offshore developments (e.g. marine renewable energy installations) change patterns of stratification, their construction may have consequences on the foraging efficiency of seabirds.