Forest and Rangeland Birds of the United States

Natural History and Habitat Use

Introduction
As North America was settled, many once-abundant birds began to disappear. Some were hunted, but habitat loss or alteration was responsible for most losses. Around the turn of the twentieth century, conservationists began efforts to preserve habitats so that birds, especially plumed wading birds, could survive. Legislation was initiated to assist in the conservation of birds and to manage hunted species. Today, many public and private conservation agencies are involved in managing bird populations and their habitats.

Birds are important components of ecosystems. Birds disseminate seeds and prey upon innumerable insect and vertebrate pests. They are involved in energy transfer as they eat and are eaten. Nutrients are also distributed through the movement of birds. Vultures, crows, ravens, and other scavenging birds are important in natural decomposition cycles. Because birds are not isolated components of our natural systems but integral parts of them, it is vital to understand both their roles and their needs.

Management of birds -- whether for human enjoyment, consumptive use, or ecological considerations -- requires data on their biology and habitat use, as well as an understanding of community interactions. Wildlife biologists, foresters, rangeland managers, and land-use planners use comprehensive information on the total community -- including birds -- in habitat management. Information must be available on the species present in the area, their habitat requirements, and how birds will respond to habitat alterations.


Distribution of Birds in the United States
There are several broad patterns in the ecological distribution of breeding birds in the United States. There is a generally increasing continuum of breeding bird species from the drier areas of the Southwest and West to the more moist forests of the Northeast. Regions dominated by desert vegetation have relatively simple avifaunas that contain a few prominent species. Regions with complex mixed forests contain many rather evenly distributed species, although much regional variation occurs along this gradient. In the Great Plains, the number of species increases from Texas to the Canadian border. This latitudinal increase may reflect patterns of glaciation or a more heterogeneous landscape on the northern plains, but in general the avian community in grasslands is organized by the most obvious structural feature, grass height.

Mountainous regions of the United States have relatively diverse avifaunas, largely because their considerable topographic relief compresses several different vegetation zones into a relatively small area, rendering them ecologically diverse. Closely related habitats, or habitats with similar physical profiles or complexities, exhibit similar bird diversities.


Importance of Vegetation Structure
An avian community, as defined here, is an aggregate of species existing together in a definable ecological area that provides the species' requirements. Each species can exist only where its specific requirements are met. Within the general habitat provisions of food, water, and shelter, birds have various specific needs for nest sites, song posts, perch sites, and vegetation structure. Some species have relatively narrow ranges of tolerance for specific factors. A prime example is the Kirtland's warbler, which breeds only in fire-regenerated stands of young jack pine in Michigan's Lower Peninsula. Others, such as the American robin, which breeds throughout North America, have broad ranges of tolerance and so are widely distributed.

Many components of the environment, including vegetation structure, plant species composition, succession, and vegetation layering, affect the distribution of bird species. What is not so obvious is that there are two basic sets of factors, ultimate and proximate, that determine whether a bird can reproduce in a given area. Many of the factors that actually determine reproductive success are not evident at the time the bird arrives or selects its breeding habitat. Keys to these ultimate factors, such as food availability for nestlings, are perceived in advance through proximate factors -- aspects of the physical habitat, especially vegetation structure.

Ever since Lack suggested that birds select breeding habitats by recognizing features they did not immediately require for survival, many studies have been conducted to identify the features or patterns of vegetation structure that bird species were "programmed" to seek. Beecher expressed a similar idea, and suggested that a bird did not "adapt" to a so-called new habitat but rather chose the habitat because of its programmed ability to recognize potentially satisfactory ultimate factors.

MacArthur and MacArthur demonstrated that the vertical complexity of forest vegetation (the diversity of vegetation heights and density of foliage at those heights) affects breeding bird diversity. The relationship of bird species diversity to foliage height diversity has been demonstrated in many forest habitat types. Foliage height diversity may be an indicator of total foliage volume. The important consideration for managers, however, is that habitat alteration changes the number of bird species and their relative abundances, both of which affect diversity.

Studies of habitat selection and resource partitioning by breeding birds include measurement of many descriptors of stand structure. These stand measurements - canopy height, layering and closure, tree diameter and species composition, understory height and volume, ground cover, etc. - are attempts to identify the proximate factors that birds select when settling on the breeding grounds.

Horizontal diversity or patchiness (the distribution of successional stages, timber size classes, and openings) is also important to breeding bird composition. Roth demonstrated that the number of bird species increased faster than the degree of species overlap in a series of habitats from grasslands to forests, and that horizontal habitat patchiness was a better predictor of the numbers of bird species than was vertical habitat complexity.

Both the vertical diversity or structure of forest stands and the distribution of stands of different size class or type are typically manipulated in forest management and can be altered as needed to manage the type and availability of bird habitat.

The close relationship between habitat structure and bird species composition is useful for assessing the effects of forest management on breeding birds. For example, as stands of northern hardwoods (sugar maple, American beech, yellow birch) develop after clearcutting, each tree size-class - regeneration, seedlings/saplings, poles, and sawtimber - supports a different breeding bird species composition. In the Willamette Valley, birds respond to successional patterns as Oregon white oak is replaced by Douglas-fir and finally by true fir and western hemlock. Downy woodpeckers, black-capped chickadees, and white-breasted nuthatches breed in the oaks, while chestnut-backed chickadees, red-breasted nuthatches, and golden-crowned kinglets commonly breed in Douglas-fir stands.

Long-term changes in bird populations occur in response to environmental change. As land uses change, or as succession proceeds, bird communities and populations will change. Most habitat management projects are, in effect, attempts to control succession: either setting it back to an earlier stage, arresting it, or allowing it to advance to a desired stage. This process can also be accompanied by short-term changes in which individuals adapt to changing conditions. Outbreaks of some insects, for example, might attract birds to a forest where they normally do not feed, such as the woodpeckers that congregate in areas of mountain pine beetle infestation.

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