Seabirds and other predators foraging in the ocean's pelagic zone face unique challenges because the distribution of their prey is often patchy and unpredictable. On the other hand, some environmental features, such as fronts, eddies, and underwater topography, do support persistent concentrations of prey and provide more predictable foraging habitats. A successful forager presumably uses both consistent and flexible strategies. However, our understanding of this balance is limited, because simultaneous high-resolution measurements of prey and predator distributions are rarely available. I used a modified marine radar to investigate the foraging behaviors of common and roseate terns (Sterna hirundo and S. dougallii) breeding at Great Gull Island, New York, USA (GGI) in 2014 and 2015. Terns are plunge-diving piscivores that often feed in dense, circling flocks, whose location and size (i.e number of birds) the radar was able to accurately record. I also surveyed the distribution of the terns' prey and the velocities of tidal currents around GGI, using scientific echosounders and an acoustic Doppler current profiler mounted on a small boat. My goal was to determine the respective roles prey density, tidal currents, and tern behavior play in generating feeding hotspots.
Results/Conclusions
Feeding flocks formed almost exclusively in particular areas upstream of tidal rips, where water was forced to flow over shallow topography and current speeds were highest. The location of these rips was quite consistent from one tidal cycle to the next, though flocks did not always form. Flocks were more common close to the island during the terns' chick-rearing period in July. Fish were significantly more abundant in 2015 than 2014, but flocks were 32% less frequent, thought they contained on average almost twice as many birds. Within each season, flock size was only weakly related to the local prey density. Flock frequency scaled with size-2 across three orders of magnitude, suggesting the process of flock growth may have scale-free or self-organizing properties. These 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 their prey. A strong role for social behaviors (e.g. "local enhancement") also appears likely. These findings will help identify important foraging habitat for terns, and could help predict their responses to changes in prey abundance.