Résumé
Seabirds foraging on pelagic fish develop behavioral strategies specifically adapted to locate inconspicuous prey that are aggregated in spatially dynamic patches. In the marine environment, they may use various mechanisms to detect cues of prey availability. The aggregation of predators at a patch of food is a particularly obvious cue to locate prey, a mechanism known as local enhancement. Pioneering studies described the formation of foraging groups at sea, showing that seabirds are attracted to feeding conspecifics. Improved foraging success due to local enhancement has been suggested from modeling studies, but no direct validation of these results exists. We deployed video cameras concomitantly with GPS loggers on Cape gannets to study the behavioral responses of equipped birds to the aggregation of predators at food patches. We showed that the reaction distances of equipped birds increased with the size of an aggregation, demonstrating that predator aggregations enhance food detectability for foragers. For small aggregations (< 50 gannets), reaction distances were mostly less than 10 km, and they increased up to almost 40 km for larger aggregations (100-150 gannets). In addition, we showed that the number and frequency of dives increased with the number of conspecifics aggregated, up to a threshold. The predator aggregations on a patch of food could, therefore, not only inform about the presence of prey but also entail information about foraging conditions. From direct observations on the various components involved, our study provides justification of the use and advantages of local enhancement in foraging seabirds.To locate inconspicuous and ephemeral fish schools in the ocean, seabirds may use indirect cues such as feeding conspecifics. Based on video cameras and GPS loggers concomitantly attached to Cape gannets, we showed that predator aggregations on a patch of food enhanced prey detectability. Gannets reacted to join a group of 50 feeding conspecifics from similar to 10 km, whereas a group of 100-150 gannets triggered a reaction from up to 40 km.