Better and faster decisions by larger fish shoals in the wild.

Studies on collective cognition provide many examples of how the efficient spread of information within groups leads to benefits with increasing group size. However, little is known if groups also amplify maladaptive information such as false alarms and whether such costs reduce possible benefits. Here, we investigated wild fish shoals responding collectively with escape dives when attacked by birds. We analyzed the collective response in reaction to bird attacks and similar but harmless flybys as a function of shoal size. Larger shoals increasingly detected predator attacks (i.e., true positives), while their response facing harmless flybys (i.e., false alarms) remained constant. Furthermore, decision time decreased with increasing shoal size. Larger shoals were thus able to simultaneously overcome two major trade-offs inherent in solitary decision-making: the trade-off between true and false positives and the trade-off between speed and accuracy. Our findings set the stage for the next generation of studies investigating the mechanisms underlying collective decision-making.