The mechanisms of interference competition: two experiments on foraging waders

• Main Authors: Vahl, Wouter K, Meer, Jaap van der, Weissing, Franz J, Dullemen, Diederik van, Piersma, Theunis
• Format: Journal Article
• Language: English
• Published: 06.07.2005

Models of population dynamics that include interference competition have often been applied to foraging waders and less so toother foragers, even though these models are, in principle, generally applicable. At present, however, it is still unclear whetherinterference competition is of importance for foraging waders. To support this idea experimental evidence and knowledge of themechanisms underlying interference effects are required. We experimentally determined the relationship between forager densityand foraging success in two wader species: the red knot (Calidris canutus) and the ruddy turnstone (Arenaria interpres). With each ofthe two species, we conducted an experiment consisting of 300 one-min trials. In these trials we scored the behavior and theforaging success of focal individuals at specific combinations of bird and prey density. Irrespective of prey density, individuals ofboth species discovered fewer prey items at higher bird densities. Despite this, only in turnstones did intake rates decline withincreasing bird density. Knots compensated for a lower prey-discovery rate by rejecting fewer prey items at higher bird densities. Inknots, bird density had a complex, nonmonotonic effect on the time spent vigilant and searching. In turnstones the main effect ofincreased bird density was a reduction in the prey-encounter rate, that is, the reward per unit search time. Effects on the timespent vigilant and the time spent searching were less pronounced than in knots. Thus, the mechanistic basis of the effects of birddensity was complex for each of the two species and differed between them.