Food divisibility and interference competition among captive ruddy turnstones, Arenaria interpres

Interference competition among foraging animals arises from agonistic interactions among foragers. Interactions can concern single food items but also clumps of food. Food clumps consist of multiple food items, and are therefore easier to divide among foragers than food items. Theoretical studies indicate that differences in divisibility can be essential to the interference process. Empirically, however, little is known about effects of resource divisibility on interference competition. Therefore, we performed an experiment with captive ruddy turnstones. Turnstones foraged either alone or together with a competitor. We offered food at two so-called food pits and varied the divisibility of food in these pits by burying a fixed number of food items either in several layers (divisible) or in a single layer (indivisible). Additionally, we varied the distance between food pits. We accounted for differences in the social dominance status of foragers by using pairs of foragers as our experimental unit: each pair had both a dominant and a subordinate member. We found a strong asymmetry in the intake of birds of different dominance status. The strength of this asymmetry depended on both the divisibility of food and on the distance between food pits. Only when food was divisible did subordinate foragers get a share of the food; only when food pits were close to each other could dominant foragers monopolize food pits. These findings imply that to understand and predict interference competition we need to consider both the detailed characteristics of resources, and the determinants of dominance status.