Cover sketch: Saw-whet owl, by Bob Hines of the U.S. Fish and Wildlife Service, Washington, D.C.

Scott, Virgil E., Keith E. Evans, David R. Patton, and Charles P. Stone.

1977. Cavity-nesting birds of North American forests. U.S. Dep. Agric., Agric. Handb. 511, 112 p.

Habitat, cavity requirements, and foods are described for 85 species of birds that nest in cavities in dead or decadent trees. Intensive removal of such trees would disastrously affect breeding habitat for many of these birds that help control destructive forest insects. Birds are illustrated in color; distributions are mapped.

PREFACE

This Handbook is the result of a cooperative effort between the Forest Service, U.S. Department of Agriculture, and the Fish and Wildlife Service, U.S. Department of the Interior. Authors Scott and Stone are wildlife research biologists with the Fish and Wildlife Service’s Denver Wildlife Research Center. Scott is stationed in Fort Collins, Colorado. Authors Evans and Patton are principal wildlife biologists with the Forest Service’s North Central Forest Experiment Station and Rocky Mountain Forest and Range Experiment Station, respectively. Evans is stationed in Columbia, Missouri, in cooperation with the University of Missouri, while Patton is stationed in Tempe, Arizona, in cooperation with Arizona State University.

Special thanks are due Arthur Singer, who graciously donated the use of his illustrations from “A guide to field identification: Birds of North America,” by Robbins, Bruun, and Zim, which are reproduced here with permission of the Western Publishing Company. The distribution maps are also reproduced with the permission of Western Publishing Company. © Copyright 1966 by Western Publishing Company, Inc.

We would like to thank Kimberly Hardin, Beverly Roedner, Mary Gilbert, Steve Blair, and Michael, Leslie, and Mary Stone for their assistance in collecting literature. A special thanks to Jill Whelan for her assistance in literature searches, checking references and scientific names, and assembling this publication. The assistance of Robert Hamre in encouraging and guiding the preparation of this manuscript is acknowledged and very much appreciated.

Cavity-Nesting Birds of
North American Forests

Many species of cavity-nesting birds have declined because of habitat reduction. In the eastern United States, where primeval forests are gone, purple martins depend almost entirely on man-made nesting structures (Allen and Nice 1952). The hole-nesting population of peregrine falcons disappeared with the felling of the giant trees upon which they depended (Hickey and Anderson 1969). The ivory-billed and red-cockaded woodpeckers are currently on the endangered list, primarily as a result of habitat destruction (Givens 1971, Bent 1939). The wood duck was very scarce in many portions of its range, at least in part, for the same reason and probably owes its present status to provision of nest boxes and protection from overhunting.

Some 85 species of North American birds (table 1) excavate nesting holes, use cavities resulting from decay (natural cavities), or use holes created by other species in dead or deteriorating trees. Such trees, commonly called snags, have often been considered undesirable by forest and recreation managers because they are not esthetically pleasing, conflict with other forest management practices, may harbor forest insect pests, or may be fire or safety hazards. In the past such dead trees were often eliminated from the forest during a timber harvest. As a result, in some areas few nesting sites were left for cavity-nesting birds. Current well-intentioned environmental pressures to emphasize harvesting large dead or dying trees, if realized, would have further adverse effects on such ecologically and esthetically important species as woodpeckers, swallows, wrens, nuthatches, and owls—to name a few.

The majority of cavity-nesting birds are insectivorous. Because they make up a large proportion of the forest-dwelling bird population, they play an important role in the control of forest insect pests (Thomas et al. 1975). Woodpeckers are especially important predators of many species of tree-killing bark beetles (Massey and Wygant 1973). Bruns (1960) summarized the role of birds in the forests:

Within the community of all animals and plants of the forest, birds form an important factor. The birds generally are not able to break down an insect plague, but their function lies in preventing insect plagues. It is our duty to preserve birds for esthetic as well as economic reasons ... where nesting chances are diminished by forestry work.... It is our duty to further these biological forces [birds, bats, etc.] and to conserve or create a rich and diverse community. By such a prophylactic ... the forests will be better protected than by any other means.

Several of the birds that nest in cavities tend to be resident (non-migrating) species (von Haartman 1968) and thus more amenable to local habitat management practices than migratory species. Nest holes may be limiting for breeding populations of at least some species (von Haartman 1956, Laskey 1940, Troetschler 1976, Kessell 1957). Bird houses have been readily accepted by many natural cavity nesters, and increases in breeding density have resulted from providing such structures (Hamerstrom et al. 1973, Strange et al. 1971, Grenquist 1965), an indication that management of natural snags should be rewarding.

Because nesting requirements vary by bird species, forest type, and geographic location, more research is needed to determine snag species, quality, and density of existing and potential cavity trees that are needed to sustain adequate populations of cavity-nesters. In a Montana study, for example, larch and paper birch snags were most frequently used by cavity-nesters (McClelland and Frissell 1975). The most frequently used trees were large, broken-topped larches (either dead or alive), greater than 25 inches diameter at breast height (dbh), and more than 50 feet tall. No particular snag density was recommended to managers. In the Pacific Northwest, Thomas et al. (1976) suggested about seven snags per acre to maintain 100 percent of the potential maximum breeding populations of cavity-nesters in ponderosa pine forests and six per acre in lodgepole pine and subalpine fir. In Arizona ponderosa pine forests, an average of 2.6 snags per acre (mostly ponderosa) were used by cavity-nesting birds (Scott, in press[1]). The most frequently used snags were trees dead 6 or more years,