Foraging hotspots of common and roseate terns: The influence of tidal currents, bathymetry, and prey density

Foraging seabirds face significant challenges because the distribution of their prey is often patchy and unpredictable. On the other hand, some environmental features do provide more predictable foraging habitats. Successful foragers presumably use both consistent and flexible strategies. However, our understanding of this balance is limited, because simultaneous highresolution measurements of prey and predator distributions are rarely available. We used a marine radar to map flocks of common terns Sterna hirundo and roseate terns S. dougallii near their breeding colony on Great Gull Island, New York, USA, in 2014 and 2015. We also surveyed the terns' prey, and tidal currents, using acoustic instruments on small boats. Feeding flocks formed most often in areas where water accelerated over shallow topography. These locations were consistent from one tidal cycle to the next, although flocks did not always form there. A spatial generalized linear model for flock density explained 45% of the deviance in their observed distribution. The mean prey density was not significantly different under flocks than elsewhere, and there were no detectable spatial trends in the prey field. Our results show that terns rely on the physical action of tidal currents to make prey available near the surface, and that the spatial and temporal predictability of this process may be more important to them than the absolute density of the prey. These findings quantify important features of tern foraging habitat, and may prove useful for planning human activities (e.g. offshore wind energy facilities) to reduce their impacts on terns.