Giese, C.L.A. 1999. Woodpecker nest site selection in oak forests of the Driftless Area in the Upper Midwest. M.S. Thesis, University of Minnesota. 69 pp.
The characteristics of woodpecker nest trees have been widely studied in some regions of North America. However, there is little research from the Upper Midwest. Knowledge of the specific characteristics of woodpecker nest trees is needed by timber harvesters to know which trees to leave during harvest to best meet the needs of cavity-nesting birds. The purpose of this study was to identify attributes of nest trees used by primary cavity-nesting birds by comparing nest trees to unused trees and by considering differences in nest trees among the woodpecker species. I found 166 active woodpecker nests in oak forests of the Driftless Area. For each nest tree, I recorded the height, diameter, status, and several aspects of tree decay (e.g., bark cover, top condition, limb condition, and presence of old cavities, branch stubs, tree scars, or significant dead portions). I also recorded these measurements of 137 randomly selected potential nest trees. Using paired t-tests and chi-square analysis, I found that each woodpecker species had a unique set of characteristics that separated their nest trees from random potential nest trees. Considered as a group, using an extension of the McNemar test for related samples, I found that woodpeckers chose trees that were larger, both in diameter and height, more often elm or aspen, more likely to have old cavities present, with more decay indicators than adjacent potential nest trees. The Yellow-bellied Sapsucker (Sphyrapicus varius) differed from the other species by nesting in living trembling aspens (Populus tremuloides) with intact tops, complete bark cover, and >3 decay indicators near the nest hole. The diameters of nest trees differed significantly among species, but height of nest hole and nest tree did not. Holes of woodpecker nest trees faced south or southeast significantly more often than by chance alone, even when excluding leaning trees. Woodpeckers create holes in trees that are used by many species of cavity dwelling wildlife. This study indicates that generic management for all woodpecker species may not be adequate because individual species have specific nest tree requirements. Management recommendations for cavity-nesting birds need to be tailored to meet a diversity of species needs.
The purpose of this study was to obtain information on the habitat surrounding woodpecker nest trees in oak forests of the Driftless Area to determine if surrounding vegetation influences nest tree selection. The influence of surrounding vegetation has ramifications for the distribution of leave trees for cavity nesting birds. It can also dictate whether management should be focused on nest trees or on broader habitat requirements. I designed the study to determine 1) if vegetation surrounding woodpecker nest trees differs from random sites and 2) if vegetation surrounding nest trees differs among woodpecker species. I surveyed vegetation in 11.3 m radius circles centered on 165 active woodpecker nest trees in oak forests of southeastern Minnesota and western Wisconsin. I recorded species, status, and size class of all trees within the circular plot. I also recorded presence and condition of potential nest trees. Additional vegetation measurements included tree canopy height, tree canopy cover, shrub cover, downed wood cover, and plot slope. I also took these same measurements in 144 randomly selected circular plots. Using Bonferronized F tests, I found many significant differences between nest sites and random sites. Forward stepwise sequential F tests indicated that the number of potential nest trees and the basal area of dead elms were the most important variables in distinguishing nest sites and random sites. Discriminant function analysis correctly classified 71% of the observations. However, when I compared nest sites only to those random sites containing a tree likely suitable for nesting, I found no differences. This suggests that the nest tree had a greater influence in nest site selection than did the surrounding vegetation. Yellow bellied Sapsucker nest trees were surrounded by a significantly higher basal area of trembling aspen (Populus tremuloides) and density of mast producing trees than the nest trees of the Downy, Hairy, Red bellied, Red headed, and Pileated woodpeckers, and the Northern Flicker. However, I found no interspecific differences among Downy, Hairy, Red bellied, and Red headed woodpeckers. My study is significant because it indicates forest management for cavity nesting birds focused on providing suitable nest trees may be more important than management focused on broader habitat requirements.