Bird functions in a forest-canopy food web were evaluated by a large-scale field experiment using ‘canopy’ enclosures. By controlling the presence of two bird species, great tits (Parus major; foliage gleaner) and nuthatches (Sitta europaea; trunk gleaner), in the enclosures, their effect on predatory insects (ants), herbivorous insects (Lepidoptera larvae) and producers (oak trees) was quantified. Great tits reduced the density of Lepidoptera larvae and, indirectly, leaf damage, but had no impact on ants. Nuthatches decreased the density of ants but did not influence either Lepidoptera larvae or leaf damage. These results highlight species-specific functions of birds in the maintenance of forest ecosystems.