Raptors
by
Mark R. Fuller
National Biological Service
Charles J. Henny
National Biological Service
Petra Bohall Wood
National Biological Service
Raptors | ||
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by Mark R. Fuller National Biological Service Charles J. Henny National Biological Service Petra Bohall Wood National Biological Service |
| Raptors, or birds of prey, which include the hawks, falcons, eagles, vultures, and owls, occur throughout North American ecosystems. As predators, most of them kill other vertebrates for their food. Compared to most other animal groups, birds of prey naturally exist at relatively low population levels and are widely dispersed within their habitats. The natural scarcity of raptors, combined with their ability to move quickly, the secretive behavior of many species, and the difficulties of detecting them in rugged terrain or vegetation, all make determining their population status difficult. | ||
| As top predators, raptors are key species for our understanding and conservation of ecosystems. Changes in raptor status can reflect changes in the availability of their prey species, including population declines of mammals, birds, reptiles, amphibians, and insects. Changes in raptor status also can be indicators of more subtle detrimental environmental changes such as chemical contamination and the occurrence of toxic levels of heavy metals (e.g., mercury, lead). Consequently, determining and monitoring the population status of raptors are necessary steps in the wise management of our natural resources. | ||
Methods |
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| We did not compile summary statistics or analyze data for any species; rather, we only have summarized the interpretations and analyses of others. Our summary of raptor status draws largely on the biological literature and on state and federal government reports. Much of this information is summarized in Johnsgard (1988), Palmer (1988), and White (1994) and in proceedings sponsored by the National Wildlife Federation (NWF 1988, 1989a, 1989b, 1990, 1991). Other information is from unpublished data (S.W. Hoffman, HawkWatch International; J.C. Bednarz, Arkansas State University; and W.R. DeRagon, U.S. Army Corps of Engineers). | ||
Selected Species |
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Ospreys |
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| Nesting ospreys (Pandion haliaetus) are concentrated along the Atlantic coast, Great Lakes, the northern Rocky Mountains, and in the Pacific Northwest. Most regional populations declined through the early 1970's, but the magnitude of decline varied, with the North Atlantic coast and Great Lakes being most severe. After the 1972 nationwide ban of the insecticide DDT, raptor productivity improved and population numbers increased in most areas. Ospreys also benefited from reservoir construction, especially in the West. Osprey numbers generally are stable, but in some areas they are still increasing. The large stick nests of ospreys, like those of bald eagles (Haliaeetus leucocephalus), are relatively conspicuous, thus aiding counts of occupied nests, which are used as a measure of population size. Counts from most states in the early 1980's provided an estimate of about 8,000 nesting pairs. Also, because several osprey populations were studied for many years, a general knowledge of their population dynamics permits a greater understanding of this species' status. | ||
Snail Kite |
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| The endangered snail kite (Rostrhamus sociabilis) breeds in central and southern Florida, the northern extent of the species' range, where it is associated with wetlands that are affected by management of water levels. From 1900 to 1960 the population declined; however, it then increased, and now remains stable with fluctuations from 300 to 800 birds (R.E. Bennetts, University of Florida, personal communication). | ||
Bald Eagles |
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| Many local bald eagle populations showed sharp declines (25% to 100%) from 1950 to the 1970's. Populations were adversely affected by shooting, habitat destruction, and organochlorine pesticides (primarily DDT). The bird was protected by the Bald Eagle Protection Act of 1940. In 1978 it was reclassified as endangered in 43 states and threatened in 5. With the documented effects of DDT on reproduction, early studies emphasized locating breeding pairs and monitoring reproductive success. | ||
| After the nationwide ban of DDT in 1972, bald eagle reproduction improved and populations began increasing. In 1981 about 1,300 pairs nested in the United States outside Alaska. The active protection of nesting habitat and release of hand-reared eagles aided the population increase. In 1993 at least 4,016 pairs of bald eagles nested in the contiguous United States, with an estimated additional 20,000-25,000 pairs in Alaska. Bald eagles nesting along the shorelines of Lakes Superior, Michigan, Huron, and Erie have lower reproductive rates and relatively high concentrations of the toxic DDE and PCB compounds (Bowerman 1993). Bald eagles nesting in Maine also have low reproductive success, probably because of environmental contaminants. | ||
| Habitat loss remains a threat in many areas. Historically there was a continuous (though scattered) distribution of bald eagles in the Southwest, south into Sonora and Baja California, Mexico, where now only a remnant population exists. Because population increases were not uniform throughout the range, the U.S. Fish and Wildlife Service has proposed downlisting this species from endangered to threatened in certain geographic areas. | ||
Hawks |
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| Populations of sharp-shinned hawks (Accipiter striatus) in the Midwest might be increasing, but analyses of eastern hawk migration count stations reveal a drop in numbers of juveniles, and blood samples collected from sharp-shinned hawks in the Northeast contained high DDE pesticide concentrations. Many other factors could be involved in a population decline, however. The sharp-shinned hawk provides an example of how monitoring can warn researchers of a potential, long-term decline in a regional population. | ||
| Similarly, the northern goshawk (A. gentilis) counts of eastern migrants suggest a stable population, but analyses of counts from the West reveal a decline. There is no widespread standardized design for surveying goshawks during the breeding season. | ||
| Habitat loss has reduced the number of Harris' hawks (Parabuteo unicinctus), whose northern range extent is the southwestern United States. Searches reveal that Harris' hawks have been extirpated from some areas such as the Colorado River Valley, California and Arizona, and that clearing of brush for agriculture likely has led to more than 50% reduction in Texas in the winter. | ||
| The biological status of the ferruginous hawk (Buteo regalis) remains uncertain because it is stable in some areas (e.g., Great Plains), but declining in other areas (e.g., half the western states). Status determination is complicated by the low density of nesting birds and fluctuations in breeding associated with cycles of prey abundance. It remains in Category 2, i.e., possibly appropriate to propose to list but available data are not conclusive for threatened or endangered status. | ||
Falcons |
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| American peregrine falcon (Falco peregrinus anatum) populations declined as a result of contamination by DDT and other organochlorine pesticides. The species was extirpated as a breeding bird in the eastern United States and declared endangered elsewhere. Peregrine recovery has been accomplished in the eastern United States and supplemented in the West (except Alaska) by release of hundreds of peregrines bred in captivity. Now several generations originating from released peregrines have survived and produced young in the wild. In some locales (e.g., parts of California), however, young are still not produced at normal rates. In Alaska nesting numbers of the Arctic subspecies increased naturally, and it was downlisted to threatened status in 1984. Now the Artic peregrine falcon is proposed for removal from the Endangered Species List. | ||
Owls |
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| The distribution of the ferruginous pygmy owl (Glaucidium brasilianum cactorum) extends north only into southern Arizona and southern Texas, and concern exists about its status because of the fragmentation and loss of deciduous riparian woodlands and remnant mesquite habitat. The subspecies occurring there, the cactus ferruginous pygmy owl, was elevated from Category 2 as of March 1993 and is being considered for listing as threatened. | ||
| The spotted owl (Strix occidentalis) is being surveyed extensively and studied because the northern and Mexican subspecies are threatened. In the Pacific Northwest the threat to these owls is loss of old-growth forest, and in the Southwest, general loss of forest habitat. The attention focused on spotted owls has resulted in the only standardized, broad-scale survey of an owl species. Since 1968 the number of known owl nesting areas in Oregon has increased from 27 records (9 sightings, 18 specimens) to about 2,700 separate sites known to be occupied by pairs or single birds sometime within the last 5 years (E. Forsman, U.S. Forest Service, personal communication). This does not reflect an increase in owls; rather, it reflects our ignorance of owl numbers and distribution, largely resulting from lack of survey effort. | ||
Conclusions |
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| Raptors, as top predators, naturally occur at low densities relative to many other organisms. As a group, raptors are poorly surveyed and there are few quantitative data with which to determine their population status and trends. A summary of our assessment of the status and population trends of the 60 species and subspecies of raptors we considered (Table) includes the following: 2 are declining in numbers and 5 are increasing; 16 (27%) are thought to be stable; 19 (32%) are classified as stable, but this assessment is qualified because of local or regional concerns or poor information; the information for 12 (20%) is so poor that we could not determine their status; 7 (12%) of these species or subspecies are endangered or threatened; and 9 (15%) are in Category 2 or 3, reflecting recent concern that they might be endangered or threatened. | ||
| We must learn more about the distribution and population dynamics of all our raptor species. With knowledge of their status and trends and information about their distribution and habitat requirements, we can avoid expensive, disruptive, last-resort management of these birds. With knowledge of their ecology, we can conserve biodiversity. | ||
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National Biological Service Raptor Research and Technical Assistance Center 3948 Development Ave. Boise, ID 83705 |
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| References | |
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Bowerman, W.W. 1993. Regulation of bald eagles (Haliaeetus leucocephalus) productivity in the Great Lakes Basin: an ecological and toxological approach. Ph.D. dissertation, Michigan State University, East Lansing. 291 pp. Johnsgard, P.A. 1988. North American owls: biology and natural history. Smithsonian Institution Press, Washington, DC. 295 pp. NWF. 1988. Proceedings of the Southwest Raptor Management Symposium and Workshop. Scientific and Tech. Series 11, National Wildlife Federation, Washington, DC. 395 pp. NWF. 1989a. Proceedings of the Western Raptor Management Symposium and Workshop. Scientific and Tech. Series 12, National Wildlife Federation, Washington, DC. 320 pp. |
NWF. 1989b. Proceedings of the Northeast Raptor Management Symposium and Workshop. Scientific and Tech. Series 13, National Wildlife Federation, Washington, DC. 356 pp. NWF. 1990. Proceedings of the Southeast Raptor Management Symposium and Workshop. Scientific and Tech. Series 14, National Wildlife Federation, Washington, DC. 248 pp. NWF. 1991. Proceedings of the Midwest Raptor Management Symposium and Workshop. Scientific and Tech. Series 15, National Wildlife Federation, Washington, DC. 290 pp. Palmer, R.S., ed. 1988. Handbook of North American birds. Vols. 4 and 5. Yale University Press, New Haven, CT. 433 pp. and 462 pp. White, C.M. 1994. Population trends and current status of selected western raptors. Studies in Avian Biology 15:161-172. |
