Variation in the strength of interference competition with resource density in blackbirds, Turdus merula

The decline in feeding rate of single blackbirds when feeding with one or two competitors was measured over a range of prey densities, in two winters, to determine whether the degree of interference competition experienced by an individual was independent of resource density. Individual wild blackbirds feeding with one or two competitors in experimental patches suffered high interference competition in both winters. In the absence of interactions (any observable despotic behaviour) there was no significant variation in the degree of interference competition with prey density in either winter. However, in the presence of interactions (an incident of chasing or being chased by a conspecific while feeding) there was significant variation in the degree of interference competition with prey density: interference competition peaked at mid-prey densities in both winters.