more than 18 inches dbh, and with more than 40 percent bark cover. The height of the snag was not as important as the diameter, but snags more than 46 feet tall had more holes than did shorter ones. Balda (1975) recommended 2.7 snags per acre to maintain natural bird species diversity and maximum densities in the ponderosa pine forests of Arizona.

Important silvical characteristics in the development of nesting cavities include (1) tree size, longevity, and distribution; (2) regeneration by sprouts, and (3) decay in standing trees (Hansen 1966). Although trees less than 14 to 16 inches dbh at maturity are too small to yield cavities appropriate for wood ducks, they may be important for smaller species. Aspen, balsam fir, bitternut hickory, ironwood, and other trees fall within this range. Short-lived species such as aspen and balsam fir usually form cavities earlier than longer lived trees. Since a major avenue of fungal infection is sprouts, sprouting vigor and the age at which sprouts are produced are important considerations in managing for cavity-producing hardwood trees. Cavity formation in oaks of basal origin is a slow process, but black oak is a good cavity producer as trees approach maturity because although the heartwood decays rapidly the sapwood is resistant (Bellrose et al. 1964). Basswood is a good cavity producer because of its sprouting characteristics. Baumgartner (1939), Gysel (1961), Kilham (1971), Erskine and McLaren (1972), and others presented information on tree cavity formation for wildlife species. More information on the role of decay from branch scars, cutting, and animal damage is needed for different species of trees so that positive management for snags may be encouraged.

Removal of snags is also known to reduce populations of some birds. For example, removal of some live timber and snags in an Arizona ponderosa pine forest reduced cavity-nesting bird populations by 50 percent (Scott[2]). Violet-green swallows, pygmy nuthatches, and northern three-toed woodpeckers accounted for much of the decline. A previously high population of swallows dropped 90 percent, and a low woodpecker population was eliminated. On an adjacent plot, where live trees were harvested but snags were left standing, cavity-nesters increased as they did on a plot where live trees and snags were undisturbed.

Foresters and recreation managers are now more aware of the esthetic and economic values of nongame wildlife, including cavity-nesting birds. In summer of 1977 the U.S. Forest Service established a national snag policy requiring all Regions and Forests to develop guidelines to “provide habitat needed to maintain viable, self-sustaining populations of cavity-nesting and snag-dependent wildlife species.” These guidelines are also to include “retention of selected trees, snags, and other flora, to meet future habitat requirements” (USDA Forest Service 1977). Some Forest Service Regions had already established policies for snag management. For example, in 1976 the Arizona-New Mexico Region (USDA Forest Service 1976) recommended that three good quality snags be retained per acre within 500 feet of forest openings and water, with two per acre over the remaining forest. The policy also requires that provisions be made for continued recruitment of future snags; spike-topped trees with cavities and obvious cull trees should be left for future cavity nesters. Some foresters are now using tags to protect the more suitable snags from fuelwood cutters in high-use areas.

In this book, we have summarized both published data and personal observations on the cavity-nesting birds of North America in an attempt to provide land managers with an up-to-date, convenient source of information on the specific requirements of these birds. Bird nomenclature follows the American Ornithologists’ Union Checklist of North American Birds (fifth edition, with supplements). Bird illustrations and distribution maps are reprinted with permission of Western Publishing Co. from A Guide to Field Identification of Birds of North America by Robbins et al. (1966). The small range maps indicate where birds are likely to be found during different seasons. Summer or breeding range is identified in red, winter range in blue; purple indicates areas where the species may be found all year. Red cross-hatching identifies areas where migrating birds are likely to be seen only In spring and fall. Length measurements (L) are for birds in their natural position, while W indicates wingspan.

Percentages of the diet under “Food” in species accounts refer to volume, unless otherwise indicated. Since nestlings of most species require insect protein, “Major Foods” refers largely to adult diets. Appendices list common names of plants and animals mentioned in the text, with scientific names when they could be determined.

Table 1.—Cavity-nesting birds: tree use and major foods.