The sandpiper’s risky business

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November 26, 2013 by comparativelypsyched

semiplmated sandpiper

When living with other people you risk a number of nasty surprises. Dirty socks on the bedroom floor! Hair clogging the shower drain! However, group living also brings a wealth of benefits, especially when you consider non-student (and non-human) living arrangements.

In a larger group there is an improved chance of spotting predators and if attacked, the odds of falling prey are decreased. Group foraging can also be more efficient as you are less likely to unnecessarily visit a food patch already exhausted by group-mates working independently. Also, if carnivorously inclined, you can cooperate to take down prey that would be otherwise unattainable (e.g. these wolves!). A recent study has demonstrated another benefit of group living.

Resources are not always exploited equally in a foraging troop. When capuchin monkeys (Cebus apella) forage, those on the periphery of the roaming group are more likely to discover new food patches, and use this opportunity to take a greater share of the food (di Betitti & Janson, 2001). Hoping to examine similar effects in a species of wading bird,  Dr. Guy Beauchamp from the University of Montreal observed groups of semipalmated sandpipers (Calidris pusilla). However, Beauchamp didn’t focus on advantages gained from individuals on the periphery. Knowing that birds in the centre of a foraging group were safer from predators, he wondered if these individuals took advantage of their relative safety to employ riskier foraging strategies. He examined two methods: Probing and skimming. Probing is the safe option; the sandpiper visually searches for signs of burrowing invertebrates and occasionally probes the sand to procure its prey. When using the skimming method the sandpiper keeps its beak close to the ground, using touch to detect smaller prey. The need to maintain contact with the sand restricts the sandpiper’s view of the surrounding beach and any potential dangers, so skimming is an inherently riskier approach. Beauchamp predicted that group foraging may encourage more flexible foraging strategies, allowing individuals to take advantage of both techniques. More specifically, he predicted that when sandpipers were found in the centre of foraging flocks they would be more likely to use the riskier method.

Calidris-pusilla-001

For one month, Beuachamp observed birds foraging on the periphery of flocks (individuals with neighbours on only one side) and birds foraging in the centre (neighbours on all sides), noting all occurrences of both techniques. The results, presented below, show marked differences in the chance a bird will use the riskier technique depending on its location in the group. In the graph below the y axis marks the probability that the skimming method is used; the bar on the left represents the central foragers while the one on the right represents those on the fringe. 

sandpiper_foraging_graph

This simple finding is striking. The relative safety at the middle of a group seems to compensate for the risky nature of the skimming method. While this is the first published example of this benefit, it is likely that similar effects occur across a range of social animals. Furthermore, Beauchamp suggests this strategy may also benefit mixed-species foraging troops. Individuals in a group consisting of more than one species take advantage of the positive aspects of group living (e.g. extra surveillance), while limiting some of the drawbacks (e.g. competition for mates). Foraging methods are often unique to a species and the safety at the centre of a mixed-species group may encourage the expression of more flexible, and more effective general foraging strategies.

Group living is complex, and it is often difficult to tease apart specific adaptive benefits. In the case of this sandpiper, group foraging affords the slight advantage of a more varied diet and an opportunity to more comprehensively exploit the fruits of the beach. It may not seem like much, but these small day-to-day advantages of group living can help minimise risk at the individual level, the level that matters most in the big picture that is natural selection.

References:

Beauchamp, G. (2013). Social foragers adopt a riskier foraging mode in the centre of their groups. Biology letters, 9(6), 20130528.

Di Bitetti, M. S., & Janson, C. H. (2001). Social foraging and the finder’s share in capuchin monkeys, Cebus apella. Animal Behaviour, 62(1), 47–56.

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