Habitat-mediated predation risk and decision making of small birds at forest edges

We studied the effect of four components of predation risk, namely predation pressure, spatial variation in woody cover, visibility, and flock size, on the behaviour of four species of the Parus guild (crested tit, P. cristatus, willow tit, P. montanus, coal tit, P. ater, and goldcrest, Regulus regulus) at the edge of patches of mature boreal forest. Birds used the exposed side of the edge (matrix) mainly during periods with low levels of predation pressure by pygmy owls (Glaucidium passerinum). Some species avoided edges under low light conditions. Birds in large groups were more prone to cross the edge, whereas group cohesion increased in risky situations, especially in the most vulnerable species (coal tit and goldcrest). The effects of these three components of predation risk were not general in that only some species responded to them, and in that intra-specific responses were not always consistent. In contrast, all behaviours examined in all four bird species (occurrence, matrix crossing, edge crossing, and group cohesion) appeared to be strongly affected by habitat-mediated predation risk. Mature boreal forests appeared to be qualitatively superior to any type of matrix in terms of protection against predators. Birds generally avoided open matrix, and seemed to move towards and across more developed stages in forest regeneration according to the ratio between food intake and predation risk attributable to a given matrix type. Open matrix (farmland and clearcuts) and very young plantations strongly restricted the rate of bird movement between old forest patches. Our results contrast with the widespread thought that birds have a great potential to use fragmented landscapes in a fine-grained manner. These limited movements across the landscape during winter might have important repercussions on the occupation of some forest patches by tits, their subsequent reproduction, and in turn their population dynamics.