Barn Owl (Tyto alba) diets were studied for 15 years in Utah. Ninety-eight percent of 111,016 prey items were mammals, heavily dominated by voles (Microtus spp.). Food-niche breadth (FNB) was 3.33 for the entire sample and varied gradually but significantly among the 15 years and among seasons. Frequency of prey in the diet did not vary significantly from year to year or among seasons. Mean daily temperatures did not vary significantly among years but annual precipitation totals and days when deep snow covered the ground varied significantly among years. Irrigation for agriculture may have partially mitigated annual precipitation fluctuations. Hay, one of the most important crops on the study area, increased over the study period and other crops decreased slightly in the amount planted. Hectares of hay planted, hectares of corn planted, and hectares of barley planted were the variables that combined to best predict annual FNB.