In conifer forests it eats seeds and fungi and stores food. It does not hibernate and remains active through the year.
The back is gray-brown and the underside yellowish-orange. Fur color and ear tufts vary with the season.
In the Cascade Range, researchers measured teeth and skulls in 70 museum specimens of Douglas squirrels, red squirrels and hybrids. They estimated bite-force quotient from lower-incisor dimensions and compared five groups from populations outside and inside the hybrid zone.
Estimated bite-force quotient was lower in Douglas squirrels than in red squirrels, both inside and outside the hybrid zone. The species did not differ significantly in mandible shape. Douglas squirrels occupy wet forests west of the Cascades, while red squirrels live in drier forests to the east, and their main conifer-cone foods differ in hardness. Each specimen group contained 10–19 animals, and skull sutures also differed among groups.
An extinction-risk assessment has not been added yet.
The wet forests west of the Cascades and dry forests to the east differ in the hardness of the squirrels’ main cone foods. Both species and hybrids occur in the hybrid zone, where the study also found differences in estimated bite force and skull traits among groups.