Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field: Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof. (Matthew 13:31-32)
More than 100 species of forest-dwelling birds can be found in the Great Lakes region. Many of these species are unfamiliar to most people because they spend most of their time in the tops of tall forest trees and are rarely seen at the backyard bird feeder.One such group of elusive woodland birds--the neotropical migrants--has been the focus of intenseconcern in recent years because of reported population declines. Neotropical migrants are birds that winter in Mexico, the Caribbean, or South America, but breed in North American temperate forests during the summer. While the population trends are not always clear, it appears that about 29 species have shown significant declines in the past thirty years and are at continued risk. About 16 species appear to be increasing and 81 species show no detectable change. Examples of species at risk include several species of warblers, thrushes, flycatchers, vireos, and even the whip-poor-will. A number of factors have been implicated in the songbird declines, but most scientists agree on two major causes: increased predation and parasitism of songbird nests in the North American breeding grounds, and conversion of forest to agricultural lands in the tropical wintering grounds. There is little evidence for conventional timber harvest in North America as a significant factor; indeed, two recent reviews of the songbird decline problem by prominent ornithologists don't even mention it as a possible cause. Nevertheless, some environmental groups have used the claim of detrimental effects as an argument for curtailing or eliminating logging on public lands. These claims have been attended by much publicity but have largely escaped careful public scrutiny. The purpose of this report is to examine the actual scientific evidence regarding timber harvest effects on birds and its implication for public and private forest management, based on more than 50 research reports by leading scientists. What About "Forest Interior Birds?" While some environmentalists acknowledge the importance of disturbance in providing essential habitat for various species, the argument is commonly raised that species requiring brushy or immature forest habitat are of little regional concern because there is already plenty of habitat for them. Rather, they say, the focus should be on preserving mature and old forest for species requiring that kind of habitat. Additional large forest preserves on public land have been proposed partly to provide habitat for a group of species known as "forest interior birds." There are, of course, species that do require relatively mature or old forest habitat, and which will generally be uncommon in areas where most of the trees have been recently harvested. Management practices on public lands do maintain large areas ofmature forest partly for this reason, and provisions for restoring old-growth characteristics are also being implemented. Discussions about "forest interior birds," however, have often been misleading for a number of reasons: • The term "forest interior birds" as used by ornithologists refers to species that cannot breed successfully in small tracts of forest surrounded by agriculture or suburbandevelopment. The common requirement among these species relates to the size ofthe forest tract, not necessarily the age. In fact, many of these "forest interior birds" are actually species such as the Kentucky warbler, Canada warbler and American redstart that prefer brushy openings or young stands of trees.
The break-up and replacement of forests by agricultural fields, roads, and suburban tracts are clearly correlated with reductions in the nesting success of many forest species. The connection has been documented in field studies made at both local and regional levels: on the smaller scale, within a woodlot as a function of distance from its edges and on the larger scale, among landscapes that vary in their amount of forest cover.
Landscape fragmentation has so strong an effect on nesting success that birds that breed within and near the edges of small agricultural woodlots may not be producing enough young to replace the adults that die each year. Such regions act as sinks in terms of the population of a given species. The continued presence of songbirds in these areas can be sustained only by immigration from places that are not so affected -- thesources, which are in the interior of large habitat tracts, far from the edge, where a surplus of young is being produced. As landscapes become more and more fragmented, the sink habitats outweigh the sources, and populations decline overall.
The problem is exacerbated for birds that prefer to nest near the edges of woodlots or fields, as these sites can be particularly hazardous. Among those that are naturally drawn to these potential "ecological traps" are Wood Thrushes, Hooded Warblers, and Summer Tanagers. More and more edges are an inevitable product of our tendency to break up larger natural areas into ever smaller ones, and the edges that we create -- such as those that bound agricultural openings -- are by design abrupt, and are purposely maintained that way. They differ considerably from the more tapered, natural edges, bordered with shrubs, that provide added cover for nesting birds. Although most field studies of the effects of fragmentation have been limited to forests and forest birds, there is growing evidence that the nesting success of grassland birds is also reduced near edges and in smaller tracts
Most bird species were affected by salvage logging (measured by snag density), a relationship that was positive for forest birds and negative for open-habitat species. Species linked to shrub and edge habitats were positively affected by vegetation regrowth. Bird communities in logged areas held more species of conservation concern than those in unlogged areas. Species richness and overall density tended to decrease from the first to the second year after fire and to increase from the second to the third.
Salvage logging benefits a number of open-habitat species, although its effect on bird conservation depends strongly on the specific threats that birds face in each region or ecosystem.
Synthesis and applications. In the Mediterranean Basin, some postfire salvage logging of pine forests can be compatible with bird conservation. We recommend that managers retain some standing dead trees during logging operations and that logged forest is interspersed with unlogged stands. This will provide suitable habitat for the widest range of species.