Rodent Prey of the Barn Owl Tyto alba and Short-Eared Owl Asio flammeus During Winter in Agricultural Lands in Southern Chile

We compared the consumption of rodent prey by barn owl Tyto alba and short-eared owl Asio flammeus during winter in agricultural areas in southern Chile. Diets were studied on the basis of pellets collected during the winters of 1986, 1987 and 1996. Both owl species consumed a large number of different rodent prey (8–9 species), but they preyed more often on the olivaceus field mouse Abrothrix olivaceus (56% and 52% of all individual prey, respectively) and long-tailed pygmy rice rat Oligoryzomys longicandatus (12.6% and 18.6%, respectively). The diet of the two owl species largely overlapped (95%). The diet diversity of barn owls and short-eared owls was not statistically different and accordingly they showed a similar evenness in diet. The geometric mean weight of rodent prey (GMWP) for short-eared owls (33 ± 1.36 g) was significantly greater than that of barn owls (28.3 ± 1.81 g). No significant difference was found between the proportion of native and introduced rodent prey consumed by barn and short-eared owls. The high diet similarity between both owl species could be a result of convergencies in hunting modes and activity time, similar body mass, or homogeneous rodent prey distribution and abundance in the agricultural areas we studied. The higher GMWP for the short-eared owl was probably caused by the fact that it preyed on the largest rodent prey in our study sites, the Norwegian rat Rattus norvegicus. According to our rodent trapping in the field, barn and short-eared owls appear to be opportunistic rather than selective predators in agricultural areas in southern Chile.

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