with group size in ostriches but not rheas. Therefore, the many eyes hypothesis does not always hold. Although individuals in groups may be able to decrease the time they spend scanning, and hence reach their energy requirements in a shorter period of time, the increased competition for food in groups may mean that animals need to devote more time overall to foraging due to an increased allocation of time for aggressive behaviour during foraging. For example, the rate of fighting between house sparrows (Passer domesticus) increases with the size of the flock. Alarm signals Large groups of animals are able to detect predators earlier because of the higher probability that at least one individual is being vigilant when the predator approaches. As many predators rely on the element of surprise in successful attacks, early detection of predators reduces predation risk. The detection of a predator by one individual only translates to collective detection if that individual makes some sort of signal to alert the rest of the group. The signal may be a deliberate call made by the vigilant individual (as in the case of meerkats) or simply the departure of the individual who has detected the predator. Flocks of birds often exhibit collective detection. One or more birds initially detect the threat, and other birds that did not perceive the threat detect their departure and also respond by fleeing. The departure of multiple birds simultaneously is likely to be a more effective alarm signal than that of a single bird as birds regularly depart flocks for reasons other than predator detection. Water skaters (Halobates robustus) transmit predator avoidance behaviour to the group through the sense of touch: individuals at the edge of the flotilla detect a predator and move, bumping into their neighbours, who in turn start moving and bumping into more individuals. This ‘wave’ of alarm has been termed the ‘Trafalgar Effect’. Sentinels In some species, individuals in a foraging group take on sentry duty. The sentinels look out for predators (often from a good vantage point) while the rest of the group forages, and sound an alarm when they spot a predator. Sentry duty is particularly important for species whose foraging activity is incompatible with vigilance, or who feed in areas where they are highly exposed to predators. For example, dwarf mongooses (Helogale parvula) dig up arthropods from the ground to eat; an activity which requires fixation