Elsevier

Animal Behaviour

Volume 69, Issue 6, June 2005, Pages 1285-1292
Animal Behaviour

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

https://doi.org/10.1016/j.anbehav.2004.10.007Get rights and content

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.

Section snippets

Escape responses and video analysis

The study area consisted of salt marsh habitat backed by woodland or dunes at Tyninghame Estuary, East Lothian, U.K. (see Whitfield 1985 for study site details). We videotaped escape responses (N = 76) from the edge of the salt marsh, using a digital video camera, on 37 days over the winter from October 2001 to February 2002. On some days more than one flock response was recorded (X¯±SE=2.05±0.20), some of which would have included the same individuals. We assumed that these responses were

First response delays

When we controlled for two significant confounding effects (temperature and number of detectors), the initial analysis suggested that the first response delay increased with spacing, supporting the PLH (Table 2). However, one outlier in the data (response delay = 30, ln spacing = 3, flock size = 2) clearly had a large influence on the spacing effect. When we repeated the analysis without this outlier, the significant (post hoc) interaction between flock size and spacing was no longer significant.

Perceptual limits or economics?

Most of the first response delay was explained by the EH because redshanks delayed for longer when in large flocks and when the risk was relatively low. Since the delay occurred in the absence of a hawk or false-threat stimulus, the likely explanation is that some redshanks attempted to gain further information about the perceived threat before escaping, even though other members (the detectors) had already initiated an escape response. That individuals in a flock responded at all in the

Acknowledgments

We thank Bobby Anderson, East Lothian District Council, the Tyninghame Estate, Mai Yasue and Sue Holt for logistical help. Anne Charmantier and Claire Devereux are gratefully acknowledged for helpful comments on the manuscript, and Dany Garant for statistical advice. Comments from two anonymous referees improved the clarity of the manuscript substantially. This work was funded by a Royal Society University Research Fellowship and a grant from the Leverhulme Trust.

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    1

    W. Cresswell is at the School of Biology, University of St Andrews, Bute Building, St Andrews KY16 9TS, U.K.

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