Northern Prairie Wildlife Research Center
Importance of Vegetation Structure
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 (1933) 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 (1942) 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 (1961) 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 (Karr 1968, Karr and Roth 1971, Willson 1974). 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 (1976) 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 (DeGraaf 1987). 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 (Anderson 1970).
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.