A review of tree-scale foraging ecology of insectivorous bark-foraging woodpeckers in North America

Woodpeckers are important forest species because they prey on tree pests and have been argued to act as keystone species through their effects as ecosystem engineers, creating breeding and shelter sites for numerous vertebrate and invertebrate taxa. We reviewed literature on tree characteristics associated with use by North American woodpeckers in the genera Campephilus, Dryobates, Dryocopus, and Picoides because forest alterations affecting this insectivorous bark-foraging guild can have cascading effects on other forest wildlife. We found that the number of studies of woodpecker tree-scale foraging was uneven across both species (ranging from 22 studies of hairy woodpeckers [Dryobates villosus] to no studies of tropical species) and ecoregions (ranging from 38 studies in Eastern Temperate Forests to no studies in Tropical Dry and Tropical Wet ecoregions). Unaltered forests were studied less than altered forests, with studies in silviculture- and wildfire-altered stands being the most commonly studied. Studies of tree characteristics largely focused on tree architecture (estimates of tree size, tree species), tree condition (live, degrading, dead, aged), foraging location (where birds foraged on trees), and environmental factors (tree burn severity, presence of bark- and wood-boring beetles, pheromone and herbicide applications), revealing four consistent patterns: 1) use of trees with relatively moderate-large diameter, 2) use of trunks more than other foraging substrates (e.g., limbs, twigs, leaves, cones, etc.), 3) use of middle portions of trees more than the lower- and upper-most portions of trees, and 4) a positive relationship between woodpecker tree use and tree use by boring beetles. Studies relating tree use to tree anatomy (e.g., bark thickness, wood hardness) and physiology (e.g., characteristics of oleoresin, phloem) were rare, including those related to tree environmental stressors (low water availability, high tree density, fire) that affect tree use by boring beetles. We emphasize the lack of studies regarding foraging ecology related to ecoregion, species, and tree characteristics as important areas for future research. We suggest land managers use studies of local foraging ecology to inform forest management when possible due to the variation in tree-scale foraging ecology among species and habitats, and provide suggestions for improving the efficiency of management-based research.