Forest and Rangeland Birds of the United States
Natural History and Habitat Use
Forest and Rangeland Management
Most forest management activities are not directed primarily at wildlife, but proper planning can help maintain or enhance wildlife habitats while accomplishing other goals. In logging or thinning operations, successional patterns are changed, creating habitat for different species of birds. Clearcutting reduces nesting, shelter, and feeding habitat for forest canopy birds, but the habitats of ground and shrub dwellers will be enhanced. Small rodents will increase in clearcuts and provide a food base for raptors. As the forest regenerates, woodland hawks and owls replace open-country hawks and falcons. Forest canopy birds will replace the ground dwellers.
Even-aged forest management creates forest stands of uniform age or size classes. The forest will have a variety of openings and stands, each with a fairly homogeneous structure, especially in smaller size classes. Thus, over the entire forest, a variety of bird habitats exists.
Uneven-aged forest management produces trees of markedly different ages or sizes in the same stand. Thus, each stand in the forest has a fairly diverse structure, but all stands are fairly similar. The result can be a more diverse bird community in each stand than in any one stand under even-aged management, but it is often less diverse than that in an entire forest under even-aged management, where many successional stages are represented. A northern hardwood forest under even-aged management, for example, supports approximately twice as many breeding bird species as does an extensive uneven-aged forest. The lack of distinct, early successional stages in the uneven-aged northern hardwood forest means that species associated with those habitats - willow flycatchers, cedar waxwings, eastern bluebirds, chestnut-sided and mourning warblers, among others - will not likely be present (DeGraaf 1987).
Deciduous Forests
A striking feature of the bird communities of deciduous forests is the high proportion - up to 75% - of migratory species. Thus the total bird population (the standing crop) is very high in the summer breeding season. Bird communities in coniferous forests are seasonally less variable. In the deciduous forest there is a general pattern of increasing bird density with plant succession (see Shugart et al. 1975 for examples). The general pattern of avian succession is generally acknowledged to be a manifestation of the habitat preferences and ecological requirements of forest birds.
In northern hardwood types, as with other hardwood types, the composition of bird species varies with timber size class, the presence or absence of softwoods in the stand, stand area, the presence of cavity trees, openings, and other within-stand features. In New England, even-aged sawlog stands have avifaunas very similar to those in uneven-aged stands because the diameter distributions in both are similar; there are essentially no differences in foliage profiles in stands more than 30 years old (Leak 1979, Aber 1979). Thus the breeding bird assemblages in stands beyond the pole stage are similar. On wetter sites, where red spruce comes into mature stands, the softwood component provides habitat for species - especially warblers - that are typically associated with coniferous types.
Oak-pine and oak-hickory types, which together compose most of the inland forests of the United States, support 150 to 200 species of breeding birds. Many are associated with successional stages and wetland and open habitats within these broad types, but their avifaunas are nevertheless quite rich. In oak-hickory types, site quality determines the dominants in mature stands: red, white, and black oak on good sites, post oak and blackjack oak on poor sites, and white and post oak on intermediate sites. Succession in oak-pine and oak-hickory types is similar, except that pine seedlings come in after disturbance during the "brush" stage in oak-pine.
Old-field successional stages are different from those after clearcutting, especially in the oak-pine type. Old fields produce essentially pure pine stands that persist a long time before oak components appear. After clearcutting, however, all components of the oak-pine types are present throughout stand development (Evans 1978).
Oak-hickory and oak-pine forests, largely due to these differences in their successional patterns, support somewhat different avifaunas. Many species associated with coniferous forests are found in the oak-pine type (see Evans 1978 for review). The larger difference between the two types is the greater value of oak-pine forests to wintering birds because the type is more southern than oak-hickory, and conifers provide additional cover.
In the western United States, aspen stands provide especially rich bird habitats compared to coniferous types. Moist ground surface, high insect populations in the understory, edge effect, and nest hole availability (depending upon woodpecker use and Fomes infection) have been identified as factors primarily responsible for the rich avifaunas in these stands (Winternitz 1976).
In both the desert Southwest and the Great Plains, riparian woodlands are essentially deciduous; cottonwoods are important in both regions, although Great Plains riparian woodlands contain more tree species as one moves eastward. These habitats contain bird communities that are as much as seven times as rich as those in the surrounding Plains habitats in general (Tubbs 1980).
In the Southwest, a variety of riparian habitats, each with a more or less unique assemblage of breeding bird species, is extremely important for wintering and migrant species also. Bird density is directly correlated with densities of cottonwoods in these wooded southwestern-riparian zones (Carothers and Johnson 1975).
Coniferous Forests
Approximately 150 bird species are associated with coniferous forests across the United States; the greatest number of species is found in mature forests in the Northeast, and the lowest in young stands in the same region. In other regions of the United States, the number of species is both lower and fairly uniform (Wiens 1975). A general pattern in coniferous forest avifaunas is that they are characterized by a few abundant species. Approximately a quarter of the individuals are of a single dominant species, and one-third to one-half belong to the two most abundant species.
The degree of dominance by a few species decreases with species richness. Several dominant species that occur throughout coniferous forests in the United States include: northern flicker, red-breasted nuthatch, brown creeper, American robin, hermit thrush, golden-crowned kinglet, dark-eyed junco, and chipping sparrow. These species occur in all coniferous regions, and can be considered the group most representative of this vegetation type across the United States (Wiens 1975).
Analyses of the ecological structure of coniferous forest avifaunas - e.g., those of Balda (1969), MacArthur (1958), and Sturman (1968a, 1968b) - have revealed several patterns of habitat utilization. Foliage-gleaning species are the most abundant birds in all North American coniferous forests, while ground-gleaners, bark-gleaners, and aerial feeders are less abundant in decreasing order.
Foliage feeders constitute the greatest proportion in mature northeastern and southeastern coniferous forests. All ground-gleaners are most important in early successional stands, and bark-gleaners and aerial feeders are proportionately more numerous in western coniferous forests.
Among foliage feeders, insectivores predominate in all forest types. Warblers constitute the major component of the breeding avifaunas of the northeastern and southeastern coniferous forests, but their relative densities are lower in the western forests. Both the number of warbler species and their densities are substantially lower in western forests than in eastern North America, and their paucity is not compensated by other foliage-gleaning insectivores. These differences likely reflect differences in prey availability between eastern and western North America (Wiens 1975).
In the longleaf-slash pine forests of the Southeastern and Gulf Coastal Plains, bird species richness can be maintained by retaining dead trees, avoiding extensive monocultures, and controlling - rather than eradicating - understory vegetation (Wood and Niles 1978).
Loblolly-shortleaf pine, a subclimax or developmental stage of oak-hickory, occurs on the Southeastern Coastal Plain and Piedmont - the old Cotton Belt. The red-cockaded woodpecker, the only endangered species closely associated with upland loblolly-shortleaf pine, occurs in clans of 2 to 10 birds in stands at least 80 years old and 35 to 160 acres in size (Meyers and Johnson 1978).
As the loblolly-shortleaf type develops, four stages each have distinct breeding bird assemblages: grassland, shrubland, pine forest, and hardwood forest (Johnston and Odum 1956). Maintaining stands in earlier stages by shorter rotation (about 35 years) is eliminating mature pine and hardwoods. Short rotations eliminate cavity trees, understory vegetation, fruits and mast, and deciduous trees. Many bird species winter in this type. Because species richness declines from early successional stands and begins to increase at about age 35, short rotations adversely affect both breeding and wintering birds (Dickson and Segelquist 1978, Noble and Hamilton 1976, Quay 1947).
The ponderosa pine type, which has the widest distribution of any pine type in North America (Little 1971), occurs in extensive stands in northern California, eastern Oregon, and the intermountain region, and as scattered islands in the Southwest and Rocky Mountains. Because stands range from savannas to mixed pine-broadleaf transition forest to pure ponderosa pine and mixed conifer stands, the type has a wide array of bird species associated with it (Diem and Zeveloff 1980). Where ponderosa pine is an important commercial tree, maintenance of snags and cull trees is important for cavity-nesting birds; a higher proportion of the avifauna is composed of cavity-nesting species in western than in eastern forests.
Where the ponderosa pine type grades into the pinyon-juniper type in the Southwest, the ecotone between the two types contains fewer bird species than does either community alone. In the pinyon-juniper type, the number of breeding species that nest in holes or forage on trunks and branches is directly related to the density of pinyon pines. In winter, bird species richness and density are strongly related to juniper berry production (Balda and Masters 1980).
The Douglas-fir region west of the Cascade Range is intensively managed because timber values are high, especially in old stands. Under natural succession, grass and shrub stages are followed by Douglas-fir, which forms dense, even-aged stands that persist for centuries. Mortality eventually opens the stand, and true climax western hemlock and western redcedar invade and an understory is reestablished. After four to six centuries without disturbance, climax western hemlock replaces Douglas-fir (Franklin and Dyrness 1973).
In intensively managed forests, fire is commonly used for slash removal and seed-bed preparation after harvest cutting. Genetically superior seeds or seedlings are planted, and herbicides, fertilizers, and pest control practices quickly produce even-aged stands of fast-growing Douglas-fir. These trees are harvested at 50 to 150 years of age. This managed succession - shortening the grass-forb and shrub stages and eliminating snags and old-growth forest - directly affects the avifaunal composition. The second (shrub) successional stage, in which approximately 40 percent of the bird species associated with the type nest, is abbreviated.
Fire affects birds indirectly by modifying their habitats. Removal of woody vegetation creates clearings with low vegetation that favors some birds. Along the border between Arizona and Mexico, for example, open-country birds such as American kestrel, roadrunner, curved-billed thrasher, harlequin quail, and chipping sparrow are most common on the Mexican side where fire control is less stringent. The species most common in Arizona are birds of brushland or dense forest, including the blue-gray gnatcatcher, black-throated gray warbler, Scott's oriole, and rufous-sided towhee (Marshall 1963).
Some species are attracted to new burns, including robins, bluebirds, several sparrows, flickers, several woodpeckers, mourning doves, and pine warblers. Prescribed fire has long been used to produce habitat for bobwhite quail in yellow pine in the Southeast (Stoddard 1963). Breeding-bird density and diversity are slightly higher in burned than in unburned chaparral (Laurence 1966). In lodgepole pine and spruce-fir forests in Yellowstone National Park, numbers of bird species increased for 25 years after fire, then begin to decrease (Taylor 1973).
In the moist, temperate coniferous forests of the Olympic Mountains, large lightning fires are less common than in Wyoming, but a similar avian response occurs over time: more species are unique to the first 20 years after fire than to stages 100 to 300 years later. Habitat for ground- or shrub-foraging species is generally enhanced in the first few years after fire, while habitat for mature-forest birds is decreased. However, during the first 1 to 3 years after fire, the bird community in coniferous forests may be more similar to that in mature forests than to the ground/shrub community (Huff et al. 1984, Bock and Lynch 1970, Bock et al. 1978), with the greatest post-fire changes in the composition of bird species occurring after about 20 years in the western United States.
Kirtland's warbler depends completely on periodic fire to maintain a very specific nesting habitat in young stands of jack pine in Michigan's Lower Peninsula. For detailed information on the effects of fire on bird habitat, see the compilations of Lotan and Brown (1984), Wood (1981), and Wright and Bailey (1982).
Management of cavity trees has become standard silvicultural practice for bird habitat management, but continued availability of contiguous old growth for the northern spotted owl remains a real concern. A detailed review of Douglas-fir management and bird species composition is provided in Meslow and Wright (1975).
Rangeland Habitats
Rangeland avifaunas characteristically have few breeding bird species compared to forests. Approximately 40 species (excluding waterfowl, raptors, and galliforms) occur with moderate frequency across major rangelands of the United States, from the shrub steppes of the Great Basin to the shortgrass, tallgrass, and mixed-grass prairies of the Great Plains (Wiens 1973, 1974). A rather small group of species characterizes each rangeland type; sparrows are present in moderate to high numbers in all rangeland habitats. There is substantial seasonal and annual variation in total bird density but not in the species composition of breeding birds. Rangeland avifaunas are often dominated by one or a few species, and high dominance is associated with low numbers of breeding species (Wiens and Dyer 1975).
Grazing and grain production are the dominant uses of rangelands. Grazing affects breeding avifaunas in various ways depending upon the intensity and rangeland type. In general, however, where grazing regimes affect vegetative composition only slightly, effects on avifaunal composition are slight. Where intensive grazing produces marked changes in vegetative composition, avifaunal composition changes markedly, usually toward that characteristic of more xeric habitats (Owens and Myres 1973, Wiens 1973). Fencing to control grazing intensity, timing, and location may create a mosaic of range conditions and therefore of bird communities of differing structure and composition.
Type conversion to remove woody vegetation and increase forage production - whether by herbicide, mechanical means, or fire - can be beneficial or detrimental to avian habitats depending upon extent, pattern, successional stages involved, and effects on special habitat needs of certain species. An example of a detrimental effect is the removal of Ashe juniper on the Edwards Plateau of Texas, which is required for nesting by the golden-cheeked warbler, a threatened species. A beneficial effect in fire-treated chaparral is increased species richness, especially where brush "islands" are retained (Bell and Studinski 1972).
The history of western livestock grazing and big game populations and their habitats have been described in detail by Wagner (1978). Generally, bison, bighorn sheep, and cattle feed more heavily on grasses; mule deer and mountain goats on shrubs and trees; and pronghorn and domestic sheep generally feed on forbs. Elk and horse feeding habitats overlap those of all of these other herbivores. Foraging by rodents, rabbits, and hares can profoundly affect rangeland vegetation, especially in desert rangelands (Norris 1950, Rice and Westoby 1978).
The preponderance of evidence indicates that grazing is generally harmful to waterfowl habitat and nesting success (Brown and Johnston 1978, Weller et al. 1958). In the Southwest, heavy grazing has also caused serious declines in populations of lesser prairie-chickens (Brown 1978), greater prairie-chickens, Montezuma quail, California quail (Leopold 1977), and northern bobwhite (Phillips et al. 1964). Intensive grazing is considered the primary factor in the decline of the Columbian sharp-tailed grouse (Miller and Graul 1980). Attwater's prairie-chicken uses grazed pastures more than ungrazed pastures because green herbaceous vegetation is made available by grazing (Kessler and Dodd 1978). In North Dakota, American bitterns, marsh hawks, and short-eared owls nest only in tall, dense ungrazed grasses and legumes (Duebbert and Lokemoen 1977).
The effects of grazing on avian habitats vary from place to place. In areas of higher precipitation, grazing may provide more habitat patchiness and so be generally beneficial to birds. In areas of low precipitation, protection from grazing may be necessary for a species that benefitted from grazing in an area of higher precipitation. Effects of soil, slope, and exposure, as well as amount and seasonal distribution of precipitation may be more important than grazing in affecting the quality of bird habitat (Ryder 1980).
Fire is an important tool in maintaining grasslands. Where fires are suppressed, grasslands may be replaced in successional stages by shrub-dominated communities, thereby providing habitat for different species of birds. For example, control of fire in the upper Midwest has reduced that habitat for sharp-tailed grouse.
Prescribed fire maintains proper cover conditions for both prairie chickens and sharp-tailed grouse in tallgrass prairie and promotes the growth of preferred subclimax foods (Miller 1963).
Creating isolated stands of trees and shrubs often enhances habitat for grassland birds. Thus riparian habitats and shelterbelts receive disproportionate use by birds. These habitats not only have their own bird communities, but are also used by grassland-nesting species as well. Cavity nesters also use riparian habitats extensively. Primary cavity users, those birds that excavate their own nesting and roost cavities, and secondary cavity nesters, which use cavities already present, are common in riparian habitats near grasslands.
Fencerows also provide shelter and nest sites for grassland birds and add to the year-round diversity of bird communities.
Little of the Northern Plains is forested, and shelterbelts have attracted bird species that not otherwise occur there. Mature shelterbelts resemble a late successional stage of the lowland hardwood forest in the north-central states. Perches for singing and hawking birds are available above the surrounding cropland or grassland. Foliage gleaners utilize the leaves of trees and shrubs. Raptors and hole-nesters are common in old shelterbelts. None of these microhabitats exist without woody vegetation. Now, with the advent of center-pivot irrigation and large grain drills and harvesters, many shelterbelts are being removed.
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