Red squirrels occur most commonly in conifer habitats where their basic year round diet is conifer seed, supplemented by seed, berries and fruits of deciduous species when available. Conifer buds and shoots are important foods in winter and spring, as are flowers in late spring and early summer. Fungi are eaten throughout the year but mostly in autumn. Grey squirrels occur most commonly in hardwood forests where their primary food is nuts and samaras of deciduous trees and shrubs. Buds, shoots and flowers are substituted in spring and early summer, along with a variety of coarse plant and animal foods in summer. Both species prefer seed, so other plant foods predominate in the diet only in places or times when seed crops are small. Seeds of the same species are used by both Red and Grey squirrels when and where available but there have been no quantitative studies of diet of both species in closely similar habitats, so food preferences within the range of foods used by both species are unknown.

Squirrels have strong preferences for particular food species, trees within a species and even between individual seeds and buds within each tree. Diet varies between individuals and between sex and age groups within squirrel populations. The determinants of these food preferences remain unknown, despite natural advantages of squirrels for testing foraging models.