Does Tree Hardness Influence Nest-Tree Selection by Primary Cavity Nesters?

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Cavity-nesting birds that excavate nest holes may be limited by the availability of suitable substrates for excavation. Suitability of trees for excavation may be influenced by substrate hardness and excavation strength of the bird. Excavation strength, in turn, may vary among bird species, causing nest-tree selection to vary among excavator species. We examined use of quaking aspens (Populus tremuloides) for nest trees as a function of tree hardness in four species of woodpeckers: Williamson's Sapsucker (Sphyrapicus thyroideus), Red-naped Sapsucker (S. nuchalis), Downy Woodpecker (Picoides pubescens), and Hairy Woodpecker (P. villosus). Hardness of trees was measured at 95 nest trees, 94 neighboring trees, and 150 random trees using a new technique described here. Other investigators have speculated that the gross external appearance of trees/snags can be used to estimate hard-ness. Hardness decreased from live trees to partly dead trees to dead trees and with increasing height in trees, but hardness was not related to other external features such as numbers of conks or percentage of the tree covered with bark. All four bird species chose nest trees that were softer than neighboring or random trees, and nest trees were softer at nests than at other heights measured. The four species selected trees of different hardness for nesting; Red-naped Sapsucker and Hairy Woodpecker chose harder trees than Williamson's Sapsucker and Downy Woodpecker. These results suggest that primary cavity nesters are sensitive to subtle characteristics of trees that reflect hardness, some of which may not be apparent in the external appearance of the trees.