Interactions between Blarina brevicauda and male-female pairs and single females of Microtus ochrogaster and single females of M. pennsylvanicus with nestlings present and between male and female M. ochrogaster and female M. pennsylvanicus without nestlings were recorded in observational arenas. Both sexes of M. ochrogaster displayed aggressive behavior towards and successfully defended the nestlings from B. brevicauda. Female M. pennsylvanicus neither displayed aggressive behavior towards nor protected nestlings from B. brevicauda. When nestlings were not present neither sex of M. ochrogaster nor female M. pennsylvanicus displayed aggressive behavior towards B. brevicauda. We conclude B. brevicauda has the potential to be an intense predator on nestling M. pennsylvanicus, but not on M. ochrogaster nestlings.

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