The intensity of interference varies with resource density: evidence from a field study with snow buntings, Plectrophenax nivalis.

Intake rates of snow buntings feeding on artificial seed patches were measured at different bird densities, for each of two different seed densities. Interference occurred in the low seed-density treatment, with intake rates declining at high bird densities. However, interference was not found in the high seeddensity treatment. The finding that the strength of interference may depend on resource density contradicts the hypothesis that the functional response is ratio-dependent (Arditi and Akçakaya 1990). The formulation for interference from Hassell and Varley (1969), and the models of Beddington (1975), Ruxton et al. (1992) and Holmgren (1995), also assume that the strength of interference is independent of resource density. The development of behaviour-based models that consider the relation between the intensity of interference, resource density and individual state may provide a more accurate description of the process of interference.