Predation by three owl species on northern pocket gophers of different body mass.

We examined the estimated sizes of Thomomys talpoides in the diets of Bubo virginianus (1,505 g), Tyto alba (437 g), and Asio otus (245 g) in north-central Oregon between 1973 and 1982. The three owl species tended to consume T. talpoides of different size and responded differently to predicted seasonal body mass changes in the T. talpoides population. On an annual basis B. virginianus preyed upon the largest individuals (x = 67.9 g), whereas A. otus preyed upon the smallest (x = 41.0 g). When compared with the predicted values for the size composition of the T. talpoides population, B. virginianus appeared to prey upon T. talpoides randomly with respect to size, whereas A. out demonstrated a high degree of selectivity, consistently preying upon relatively small individuals. However, the absence of significant seasonal changes in prey size electivities suggests that active choice is not a major factor in predation on gophers by A. otus. Instead, evidence suggests that A. otus attacks T. talpoides randomly with respect to size but captures them in accordance with size-related capture probabilities. Tyto alba preyed most commonly upon intermediate sized T. talpoides (x = 63.3 g), and the presence of T. alba coincided with the seasonal occurrence of T. talpoides of this general size.