House mice arrived in Oregon with European settlement and NatureServe still classifies the species as exotic rather than assigning it a conservation rank. They thrive around farms, barns, and disturbed ground, so records near the Willamette Valley’s wildlife refuges more likely trace back to nearby agricultural buildings and field edges than to the wetlands those refuges are named for.
Records climb into a September and October peak, roughly three times the summer low, a pattern that likely reflects populations spilling out of buildings and fields as harvest activity disturbs their cover rather than any true fall breeding surge. House mice breed year-round indoors, so how visible they are outdoors depends heavily on the season and how much ground gets disturbed.
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