Courtesy J.C. Leupold, NBS
by
Science Editor
Russell J. Hall
National Biological Service
Division of Research
Washington, DC 20240
|
Courtesy J.C. Leupold, NBS |
by Science Editor Russell J. Hall National Biological Service Division of Research Washington, DC 20240 |
|
Birds | ||
| Overview | ||
| Migratory bird populations are an international resource for whic h there is special federal responsibility. Moreover, birds are valued and highly visible components of natural ecosystems that may be indicators of environmental quality. Consequently, many efforts have been directed toward measuring and monitoring the condition of North America's migratory bird fauna. The task is not an easy one because the more than 700 U.S. species of migratory birds are highly mobile and may occur in the United States during only part of their annual cycle. Some species annually make round-trip migrations spanning thousands of kilometers or miles, others engage in short or irregular migrations of tens or hundreds of kilometers, and even resident species are capable of moving great distances over short intervals. One often cannot tell whether a bird observed at a given moment is a resident, a migrant, a visitor from another locality, or the same individual seen 10 minutes earlier. | ||
| Determining status and trends is further complicated by the fact that each of these species has its own patterns of distribution and abundance, and each species has populations that respond to different combinations of environmental factors. Finally, the sheer abundance of birds estimated at 20 billion individuals in North America at its annual late-summer peak (Robbins et al. 1966) may make it difficult to obtain accurate counts of common species, and the absolute abundance of some may mask important changes in their status. | ||
| Biologists have developed many different approaches to determining abundance and trends in abundance, and nearly all of the recog-nized census methods applicable to birds are represented by the articles in this section. Not surprisingly, trends among the large number of populations treated are mixed. | ||
| Results from the nationwide Breeding Bird Survey (Peterjohn et al., this section) and a portion of the large-scale Christmas Bird Count (Root and McDaniel, this section) show that some populations are declining, others increasing, and many show what appears to be normal fluctuations around a more or less stable average. Overall, approximately equal numbers of species appear to be increasing and decreasing over the past two to three decades. Groups of species with the most consistent declines are those characteristic of grassland habitats, apparently reflecting conversion of these habitats to other types of vegetative cover. | ||
| Waterfowl populations are monitored closely as a basis for regulating annual harvests at levels consistent with maintenance of populations. Goose populations (Rusch et al., Hestbeck's "Canada Geese," Hupp et al., all this section) have shown some impressive gains over the past decades, but most gains have been registered by large-bodied geese, with several smaller species and smaller subspecies of the highly variable Canada goose (Branta canadensis) having depressed populations. | ||
| Censusing and determining the status of natural Canada goose populations are made more difficult by the widespread introduction and establishment of resident goose populations, which breed outside the traditional Arctic nesting areas and mix with migratory populations on the wintering grounds. | ||
| Duck surveys address more than 30 species that might be legally hunted. Even though some species are stable or even increasing, many duck populations have declined in the past decade (Caithamer and Smith, this section). Biologists attribute these declines to losses of breeding and wintering habitats and a long period of drought in breeding areas. Among species receiving special emphasis, canvasbacks (Aythya valisineria; Hohman et al., this section) showed a complex pattern with regional changes in distribution and abundance, and pintails (Anas acuta; Hestbeck's "Decline of Northern Pintails," this section) showed a widespread and nearly consistent pattern of decline. | ||
| Results are preliminary, but two new census programs, the MAPS and BBIRD programs (Martin et al., this section), promise to provide much higher quality information on status and trends by measuring not only the presence of bird populations in breeding areas, but also their success. When fully operational, this approach may offer important clues regarding the causes of observed population changes. | ||
| Shorebirds are highly migratory, and status and trends of their populations are largely determined from observations made during periods in their life cycles in which birds congregate in limited breeding, staging, or migratory stopover areas. Populations of eastern (Harrington, this section) and western (Gill et al., this section) species show general patterns of decline, although some species, including those using inland areas, are too poorly studied to detect trends. Apparent dependence on critical breeding and staging areas suggests that populations of many species are vulnerable to habitat loss and disturbance. | ||
| Seabirds in the Pacific region (Carter et al., Hatch and Piatt, both this section) include many diverse species that respond differently to factors such as human proximity to nesting areas, oil spills, introduction of predators, depletion of fishery stocks, and availability of human refuse as food. Some species, including certain gulls, brown pelicans (Pelecanus occidentalis), and double-crested cormorants (Phalacrocorax auritus), have responded positively to recent changes in some areas, whereas others, including murrelets and murres (Family Alcidae) and kittiwakes (Genus Rissa), have shown declining trends. Populations of other species appear to fluctuate widely, and information for many species is insufficient to determine long-term trends. | ||
| Colonial waterbirds of the continental and east coast regions of the United States (Erwin, this section) show trends related to many of the same factors operating in the Pacific region, with some species recovering from past losses from pesticides while some other species that exploit human refuse are increasing dramatically. Populations of other species, especially certain terns, are declining, probably as a result of habitat loss and degradation or other kinds of human disturbance. Special efforts have been made to determine status and trends of the piping plover (Charadrius melodus; Haig and Plissner, this section), a species listed as endangered in certain parts of its range and as threatened in others. | ||
| Populations of raptors (Fuller et al., this section) are difficult to census, but ospreys (Pandion haliaetus), bald eagles (Haliaeetus leucocephalus), and peregrine falcons (Falco peregrinus) have increased in numbers as they recover from past effects of pesticides. Populations of most vultures, hawks, and owls are either poorly known or believed to be stable. Notable exceptions are California condors (Gymnogyps californianus; Pattee and Mesta, this section), the crested caracara (Caracara plancus; Layne, this section), and spotted owls (Strix occidentalis), all of which enjoy or have been considered for additional protection. Mortality factors of eagles (Franson et al., this section) have been monitored and, although these data do not directly measure population status, they do indicate trends in the kinds of factors that tend to depress population growth. | ||
| The wild turkey (Meleagris gallopavo; Dickson, this section) has shown dramatic increases in distribution and abundance in recent decades because of translocations, habitat restoration, and harvest control. Mourning doves (Zenaida macroura; Dolton, this section) have shown generally stable populations, although recent population declines in the western states are disturbing. Regional increases of ravens (Corvus corax) in the southwest (Boarman and Berry, this section) are primarily of concern because of their potential effects as predators on eggs and young of the desert tortoise (Gopherus agassizii). | ||
| Populations of severely endangered species, like the California condor (Pattee and Mesta, this section), the Mississippi sandhill crane (Grus canadensis pulla; Gee and Hereford, this section), and the Puerto Rican parrot (Amazona vittata; Meyers, this section), are reasonably well known. Through censusing these species, biologists have tracked declines, often to a few individuals, and slow recoveries resulting from intensive management activities. Other rare species have populations that are depleted or vulnerable because of recent trends, but which can be censused with far less certainty. For example, willow flycatchers (Empidonax traillii; Sogge, this section) breed sparsely in parts of the Grand Canyon where exotic species have displaced natural riparian vegetation; likewise, the status of the red-cockaded woodpecker (Picoides borealis) appears closely tied to the decline of the longleaf pine (Pinus palustris) ecosystem (Costa and Walker, this section). | ||
| Broad-scale programs such as the Breeding Bird Survey, annual waterfowl surveys, and wintering surveys such as the Christmas Bird Count may provide information on status and trends for as many as 75% of U.S. bird species, at least to the extent that they would provide evidence of catastrophic declines. Remaining species may be censused only with difficulty and often with imprecision because they are secretive, rare, highly mobile, or occupy poorly accessible areas. Specialized surveys provide information on some of these groups but, as indicated by the articles in this section, they do so with varying degrees of success. Much work remains to be done on obtaining better information and developing better ways of interpreting available information on difficult-to-census species. | ||
| If any overall conclusion is possible on the wide array of information now available on status and trends of bird populations it is this: apparent stability for many species; increases in some species, many of which are generalists adaptable to altered habitats; and decreases in other species, many of which are specialists most vulnerable to habitat loss and degradation. | ||
| References |
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Robbins, C.S., B. Bruun, and H.S. Zim. 1966. Birds of North America. Golden Press, New York. 340 pp. |