Sex Ratio of Rodents as Barn Owl (Tyto alba) Prey

Sex ratios of rodents in samples of owl pellets have been interpreted to reflect predator selectivity, availability of prey on the landscape, and variable susceptibility of the sexes to predation. Metric distinction of male from female innominates of deer mice (Peromyscus maniculatus) and voles (Microtus spp.) extracted from ∼120 pellets cast by barn owls (Tyto alba) in southeastern Washington state revealed more females than males. The sex ratio did not change from pellets cast when much of the landscape was productive agricultural land to pellets cast when a portion of the land was taken out of production and placed in soil bank. Barn owls are opportunistic foragers that have greater success at prey capture when vegetation cover is discontinuous. Barn owls likely captured more young females than males of both rodent taxa because the former are more subject than the latter to intra- and inter-specific displacement into more open habitats, whether agricultural land or soil bank land, therefore are more susceptible to predation.