Escape response delays in wintering redshank, Tringa totanus, flocks: perceptual limits and economic decisions

Variation in escape response delays can be explained by at least two, nonmutually exclusive hypotheses: (1) the perceptual limit hypothesis, where delays result because of physical constraints related to predator detection and alarm signal transmission, and (2) the economic hypothesis, where delays are adaptive because they help to avoid superfluous, or to optimize essential, escape responses. We explored the relative importance of these effects in determining first response delays (the time elapsed between the 'detectors' leaving and the start of the rest of the flock's escape) and main response delays (time taken for all nondetectors to escape) among redshank flocks to three stimuli that posed different levels of risk (an attacking hawk, an approaching harmless species and a stimulus that did not involve any obvious external threat). There was strong support for the economic hypothesis because first response delays increased with flock size during responses to low-risk stimuli, when we assume the cost of not escaping immediately was lower because of increasing benefits from the dilution and confusion effects. There was also strong support for the perceptual limit hypothesis because main response delays were explained entirely by spacing and got quicker as flock size increased. This suggests that, once the rest of the flock started to respond to the detectors, benefits gained through the dilution and confusion effects decreased rapidly, so that immediate escape was the optimal response. Escape response delays can therefore be explained by both hypotheses and we discuss the implications of our results for group-living theory.