_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, 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.