The state's Fish and Game Commission voted unanimously on Friday to add the threatened bird to the list, ending a four-year process by the Environmental Protection Information Center, or EPIC.
EPIC Program Director Tom Wheeler called it a "small step toward recovery."
Scientists say that owl numbers are now dropping at an annual rate of 3.8 percent. Five years ago, the rate was 2.8 percent.
The northern spotted owl was labeled as a "threatened species" under the Federal Species Act in 1990 but the owls' population has continued to decline. Experts say changing the owl's status from threatened to endangered could lead to efforts to increase owl habitat on federal lands.
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