Part 1

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

Cavity-Nesting Birds of North American Forests

Virgil E. Scott Denver Wildlife Research Center

Keith E. Evans North Central Forest Experiment Station

David R. Patton Rocky Mountain Forest and Range Experiment Station

Charles P. Stone Denver Wildlife Research Center

Illustrated by Arthur Singer

Agriculture Handbook 511 November 1977

Forest Service U.S. Department of Agriculture

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402

Stock No. 001-000-03726-9

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, 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.

Common Name Page Snag or Major Foods[4] No. Tree Use[3] A B C 1 2 3 4 5 6 7 Black-bellied whistling 7 X X X duck Wood duck 8 X X X X X X Common goldeneye 9 X X X X X Barrow’s goldeneye 10 X X X X X Bufflehead 11 X X X X X Hooded merganser 12 X X X X X X Common merganser 13 X X X X X Turkey vulture 14 X X X Black vulture 15 X X X Peregrine falcon[5] 16 X X X Merlin 17 X X X X X American kestrel 18 X X X X X Barn owl 19 X X X Screech owl 20 X X X X Whiskered owl 21 X X X Flammulated owl 22 X X X Hawk owl 23 X X X Pygmy owl 24 X X X X Ferruginous owl 25 X X X Elf owl 26 X X X Barred owl 27 X X X Spotted owl 28 X X X X X Boreal owl 29 X X X Saw-whet owl 30 X X X X Chimney swift 31 X X Vaux’s swift 32 X X X Coppery-tailed trogon 33 X X X X Common flicker 34 X X X X Pileated woodpecker 35 X X X X Red-bellied woodpecker 36 X X X X X Golden-fronted 37 X X X woodpecker Gila woodpecker 38 X X X Red-headed woodpecker 39 X X X X X Acorn woodpecker 40 X X X X X Lewis’ woodpecker 41 X X X X X Yellow-bellied sapsucker 42 X X X X X Williamson’s sapsucker 43 X X X X Hairy woodpecker 44 X X X X Downy woodpecker 45 X X X X Ladder-backed woodpecker 46 X X X Nuttall’s woodpecker 47 X X X Arizona woodpecker 48 X X X X X Red-cockaded woodpecker 49 X X X X White-headed woodpecker 50 X X X Black-backed three-toed 51 X X X woodpecker Northern three-toed 52 X X X woodpecker Ivory-billed 53 X X X X woodpecker[5] Sulphur-bellied 54 X X X flycatcher Great-crested flycatcher 55 X X X Wied’s crested 56 X X X flycatcher Ash-throated flycatcher 57 X X X X Olivaceous flycatcher 58 X X X Western flycatcher 59 X X Violet-green swallow 60 X X X Tree swallow 61 X X X Purple martin 62 X X X Black-capped chickadee 63 X X X Carolina chickadee 64 X X X Mexican chickadee 65 X X Mountain chickadee 66 X X X Gray-headed chickadee 67 X X Boreal chickadee 68 X X X X Chestnut-backed 69 X X X X chickadee Tufted titmouse 70 X X X Plain titmouse 71 X X X Bridled titmouse 72 X X White-breasted nuthatch 73 X X X Red-breasted nuthatch 74 X X X Brown-headed nuthatch 75 X X X Pygmy nuthatch 76 X X Brown creeper 77 X X X House wren 78 X X Brown-throated wren 79 X X X Winter wren 80 X X Bewick’s wren 81 X X Carolina wren 82 X X Eastern bluebird 83 X X X Western bluebird 84 X X X X Mountain bluebird 85 X X X X Starling 86 X X X X X X X Crested myna 87 X X X X Prothonotary warbler 88 X X X Lucy’s warbler 89 X X House sparrow 90 X X X X X X European tree sparrow 91 X X X

Black-bellied whistling duck _Dendrocygna autumnalis_ L 13″ W 37″

Habitat: Black-bellied whistling ducks (tree ducks) are found regularly in southern Texas and erratically elsewhere. Open woodlands, groves or thicket borders where ebony, mesquite, retama, huisache, and several species of cacti are dominant in freshwater habitat are preferred (Oberholser 1974, Meanley and Meanley 1958). Range extensions have been facilitated by flooding and impoundments.

Nest: Natural cavities in trees such as live oaks, ebony, willow, mesquite, and hackberry are preferred, but ground nests and nest boxes are sometimes used. The nest can be over land or water, but herbaceous vegetation under “land-bound” nests may be preferred to brush (Bolen et al. 1964). A perch near the cavity entrance may also be a factor in nest tree selection. Open and closed cavities are used. Nest cavities average 8.7 feet above ground or water and 23 inches deep, with 7.0 × 12.5 inch openings (Bellrose 1976). Nesting boxes should be 11 × 11 × 22 inches high at the front and tapered to 20 inches in the rear, with entrances 5 inches in diameter (Bolen 1967). Nest boxes should not be erected unless they are predator proof.

Food: Black-bellied whistling ducks are predominantly grazers (Rylander and Bolen 1974), but they can dabble and dive for aquatic food. Of 92 percent plant materials, sorghum and Bermudagrass predominated, with smartweeds, millets, water stargrass, and corn also occurring in one study (Bolen and Forsyth 1967). In some areas corn and oats are more important in the diet.

Wood duck _Aix sponsa_ L 13½″ W 28″

Habitat: Wood ducks are associated with bottomland hardwood forests where trees are large enough to provide nesting cavities and where water areas provide food and cover requirements (McGilvrey 1968). Requirements may be met in several important forest types, all of which must be flooded during the early nesting season: (1) southern flood plain, (2) red maple, (3) central flood plain, (4) temporarily flooded oak-hickory, and (5) northern bottomland hardwoods.

Nest: Optimum natural cavities are 20 to 50 feet above the ground with entrance holes of 4 inches in diameter, cavity depths of 2 feet, and cavity bottoms measuring 10 × 10 inches (McGilvrey 1968). Management for cavities more than a half mile from water is not recommended, and dead trees, other than cypress, do not usually contain usable cavities. Good densities of suitable wood-duck cavities have been recorded for many timber types (Bellrose 1976). Nest boxes are readily used by wood ducks, and their use may increase breeding populations, even if natural cavities are abundant, if predators are excluded. Measurements and placement of wood duck boxes have been well described (U.S. Fish and Wildlife Service 1976, Bellrose 1976, McGilvrey 1968).

Food: Wood ducks consume large quantities of acorns, usually in flooded bottoms. Other mast and fleshy fruits also are eaten, as are waste corn and wheat (Bellrose 1976). Smartweed, buttonbush, bulrush, pondweed, cypress, ash, sweet gum, burweed, and arrow arum seeds are used by breeding birds. Skunk cabbage, coontail, and duckweed are also food items. Duckweed is also habitat for invertebrates in the diet (Grice and Rogers 1965).

Common goldeneye _Bucephala clangula_ L 13″ W 31″

Habitat: The breeding range of the common goldeneye generally coincides with the boreal coniferous forest in North America (Johnsgard 1975, Bellrose 1976). In a Minnesota study, 87 percent of breeding goldeneyes were found on large, sand-bottomed fish lakes (Johnson 1967), while in New Brunswick, this species preferred water areas with marshy shores and adjacent stands of old hardwoods (Carter 1958). In Maine, nests are found in mature hardwoods adjacent to lakes with rocky shores, hard bottoms, and clear water. Shoal waters less than 10 feet deep with an irregular shoreline provide brood shelter and protective vegetation necessary for duckling food (Gibbs 1961).

Nest: Common goldeneyes used and were more successful in open top or “bucket” cavities than in enclosed cavities in New Brunswick (Prince 1968). Most nests were in silver maples on wetter sites or American elms on drier sites and aspen in northern conifer forests. Nest trees averaged 23 inches in diameter with cavity dimensions of 8 inches in diameter and 18 inches deep; most entrances were 6 to 40 feet above ground (Prince 1968). Wooden nest boxes measuring 12 × 12 × 24 inches with elliptical entrances 3½ × 4½ inches were used extensively in Minnesota (Johnson 1967).

Food: Of 395 stomachs examined by Cottam (1939), crustaceans (32 percent), insects (28 percent), and molluscs (10 percent) were primary animal foods (total, 73.9 percent). Crabs, crayfish, amphipods, caddisfly larvae, water boatmen, naiads of dragonflies, damselflies, and mayflies were also found. Pondweed, wild celery, and seeds of pondweed and bulrushes were important plant materials.

Barrow’s goldeneye _Bucephala islandica_ L 13″ W 31″

Habitat: Barrow’s goldeneyes attain their highest breeding population levels in western North America on moderately alkaline lakes of small to medium size in parkland areas. Open water is a necessity throughout the range, but frequently goldeneyes favor a dense growth of submerged aquatics such as sago pondweed and widgeon grass. The abundance of aquatic invertebrates may be more important than nesting cavities in determining distribution (Johnsgard 1975).