Marine radar reveals novel group foraging patterns in pelagic seabirds.

Many seabirds are known to frequently feed within multi-specific feeding flocks. These aggregations bring facilitation to prey access as well as potential exacerbated competition. Once a feeding opportunity is found, birds rapidly aggregate to it through processes known as local enhancement. At a larger scale, seabirds may distribute themselves forming patterns such as rakes or clusters, which are thought to increase the probability to cue on other foragers. The importance of such patterns in the foraging ecology of seabirds is yet to be evaluated empirically. Using a large dataset of radar images from the eastern tropical Atlantic, spanning between November 2015 and November 2018, I visually looked for the presence of collective patterns in seabirds' distribution at sea. I showed that clusters, rakes and the newly discovered chessboard patterns were found in 75.3, 68.6 and 51.0% of the recorded days, respectively. Furthermore, these patterns were observed virtually everywhere in the eastern tropical Atlantic and preceded the observations of local enhancement. These results indicate that collective patterns represent a widespread strategy, probably involving many different species associated with subsurface predators. The significance of this ubiquity is discussed in light of the strategic thinking possibly required to maintain these patterns, as well as their evolutionary and conservation implications. This article is part of the theme issue 'Multi-predator feeding aggregations in pelagic waters'.