The effects of dominance on social foraging tactic use in house sparrows

We investigated whether social foraging tactic use (producing and scrounging) in birds is affected by the dominance rank of individuals as predicted by a phenotype limited producer-scrounger game. In a captive flock of house sparrows, we observed the behaviour of the birds when they were foraging on a grid containing clumps of seeds. We measured the fighting success of the birds, determined the method by which they found food clumps (finding or joining), and measured their feeding rate. Joining were frequently observed and usually involved aggressive interactions. Most birds used both finding and joining to obtain food. We found that foraging method was related to dominance: the frequency of joining gradually increased with increasing dominance rank, as predicted by the phenotype limited model for flocks where there are moderate competitive asymmetries among the birds. Food intake rate of individuals was not related to either their dominance rank or foraging method. Similar weak relationships were predicted by the model among these variables for flocks with moderate competitive asymmetries. Behavioural variability among sparrows in locomotion frequency and vigilance was not related to their foraging method, but the rate of investigating potential food caches strongly decreased with increasing frequency of joining. We conclude that the phenotype limited model successfully predicts the relationship between dominance and joining frequency in house sparrows feeding on concentrated food sources, and we suggest that the distribution of food used in tests of the model may crucially affect their results.