Resource availability and distribution are hypothesized to be the primary ecological mechanisms driving variation in avian mating systems. Although food supplementation experiments have been used to examine plasticity in mating strategies, variation in avian mating strategies across naturally occurring gradients of prey availability has rarely been examined. Here, we test the hypothesis that the polygynandrous mating system of Bicknell's Thrush (Catharus bicknelli) results from food limitation on female home ranges in the species' harsh montane breeding environment. In 2000, number of chicks fledged per nest was significantly positively correlated with prey biomass on a female's home range, and number of male feeders at a nest was significantly negatively correlated with prey biomass. Neither relationship was significant in 2001, a year with high nest-predation rates, which suggests that nest predation may ultimately limit the species' productivity. These results are consistent with the hypothesis that (1) female productivity is limited by prey biomass, and (2) female mating decisions may be influenced by their assessment of home-range food abundance prior to egg laying.