Sapsucker

Abstract: Nest-holes created by woodpeckers or natural decay processes are an essential commodity for secondary hole-nesting species. Hole-making agents may strongly influence the richness and abundance of species in hole-nester communities. However, few studies have examined the characteristics and relative importance of naturally occurring holes for hole-nesters. Between 1995 and 2006, we examined 1371 excavated and non-excavated holes used by 29 bird and mammal species in central British Columbia, Canada. Excavated holes were much more abundant (85%) than non-excavated holes (15%). Red-naped sapsucker (Sphyrapicus nuchalis) and northern flicker (Colaptes auratus) excavated 52% of the holes monitored. At the community level, non-excavated holes were used for nesting less than expected based on their availability (6% of total nests), particularly among woodpeckers (2% of nests). However, secondary hole-nesters used non-excavated holes roughly in proportion to their availability (10% of nests), and some excavators used non-excavated holes for nesting (flicker and red-breasted nuthatch, Sitta canadensis, 4% of nests each; black-capped chickadee, Poecile atricapillus, 13% of nests). Although nests in non-excavated holes tended to be lower on the tree, larger internally and with larger entrances, only European starling (Sturnus vulgaris) appeared to select non-excavated holes with characteristics most similar to their preferred excavated holes. Non-excavated holes may be an alternate nesting resource for secondary hole-nesters that cannot acquire an excavated hole and, occasionally, for excavators. The use of non-excavated holes may provide an advantage for secondary hole-nesters by releasing them from the constraints of excavator nest-site preferences.

Abstract: 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 hardness. 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.