Wood density and tree size used as cues to locate and excavate cavities in two Colaptes woodpeckers inhabiting a threatened southern temperate forest of Argentina

Abstract

Nest-site selection is crucial for birds' breeding success and, therefore, studies on this topic are critical in order to understand what features species need. This is particularly important in areas with conservation issues, where habitats and/or trees with the required features may be scarce. However, little information is available on the nest-site selection patterns of woodpeckers breeding in southern temperate forests, where tree logging considerably reduced forest cover and, hence, availability of trees suitable for cavity excavation. In addition, previous studies did not include assessments at different spatial scales nor including quantitative wood density data. Therefore, the main objective of this study is to assess nest-site selection patterns (at the habitat and cavity-tree scales) of the Campo Flicker Colaptes campestris and the Green-barred Woodpecker C. melanochloros breeding in a threatened southern temperate forest of central-east Argentina. Moreover, we report the first wood density data of the trees used by these two woodpeckers when excavating cavities and assess niche partitioning by comparing their cavities and cavity-trees to one another. To assess selection at a habitat scale (recording forest cover and edge of stands and trees), we compared selected breeding habitats to randomly found habitats. At a tree-scale (recording tree height, diameter, wood density and wood decay status), we compared cavity-trees to randomly found trees. Most cavities were excavated in medium (DBH = 20–35 cm) or large (>35 cm) size trees with wood density < 0.5 g/cm3. Generalized linear models indicated that woodpeckers’ propensity to excavate a new cavity was negatively correlated with tree wood density (which was negatively correlated to wood decay status) and positively to tree size. Both woodpecker species focused more importantly on tree features compared to habitat features when excavating cavities. Because forest patches containing mid- and large-sized trees with main stems and/or branches with wood softened by decay processes have become less abundant in these forests due to uncontrolled logging actions, we emphasize the need to conserve trees with these characteristics to ensure the availability of the preferred features of these woodpeckers.