We collected 621 pellets from 38 boxes. Pocket gophers and voles were the most frequent prey in barn owl diets. Deer mice and house mice were also common in diets. Birds and rats were occasionally eaten, and rabbits, insects, crayfish, bats and moles were eaten rarely. Gophers were eaten most often during the spring and summer, whereas voles were eaten most often during the winter.
Body mass is strongly related to mandible length.
In our study, mandible length averaged 2.0 cm., corresponding to an estimated body mass of 61 grams. Because the median body mass of adult gophers exceeds 90 grams for females and 120 grams for males, barn owls were evidently eating mostly juvenile gophers. Among the 6 collection intervals, gopher size was negatively correlated with percent occurrence of gophers in the diet. The number of gophers eaten per nesting pair of owls varied greatly and averaged 152 gophers for the nesting season, about 22 gophers per month per pair. This represents a minimum figure, since barn owls may regurgitate pellets away from nest boxes where we did not find them.