Tuesday, July 23, 2002

How Many Birds Are There?

Historical Estimates

McAtee (1931) estimated that there were about 2.6 billion breeding land birds in the United States.

Peterson (1948), using figures from the Breeding Bird Census, estimated the total population of breeding land birds in the Unted States at not less than 5 billion birds and perhaps closer to 6 billion birds in the beginning of summer.

Using a more complicated method than Peterson, Leonard Wing (in Terres 1980:748) estimated a summer population of 5.6 billion birds in the United States.
It is important to point out that all of these estimates, apparently, were of breeding land birds only. For the most part, they did not incorporate breeding populations of waterbirds (e.g., watefowl, colonial-nesting waterbirds, marsh-nesting birds) and almost certainly underestimated breeding populations of crepuscular and nocturnal birds (e.g., goatsuckers and owls). They did not account for coastal-nesting seabirds (e.g., pelicans, storm-petrels, cormorants, gulls, terns, alcids). They also did not account for the large populations of shorebirds that migrate through the contiguous United States to nest in Alaska and Canada, or the immense numbers of shearwaters and other tubenoses that migrate through coastal waters enroute to and from breeding grounds in the Southern Hemisphere.

Recent Estimates

Based on the detection of an average of about 3,325 birds/square mile on the 1973 Breeding Bird Survey, Aldrich et al. (1975) estimated 9.975 billion breeding landbirds for the United States exclusive of Alaska and Hawaii, and concluded that the autumn population was probably about double that figure, or around 20 billion birds.

Banks (1979) used a figure of about 10 billion breeding birds in the contiguous United States in his assessment of the relative impact of different sources of mortality, and assumed an average annual mortality of about 10 billion birdes.

I examined Breeding Bird Censuses for 1991 and 1992 (J. Field Ornithol. 63 [Suppl.] and 64 [Suppl.]. The median breeding densities on plots censused were 220 pairs/40 hectares in 1991 (n = 126) and 206 pairs/40 hectares in 1992 (n = 132). Extrapolating these figures to pairs/square mile (1,430 and 1,334, respectively) and expanding to the land area of the contiguous United States (2.966 million square miles), the two samples yield estimated poplations of 8.4 and 7.9 billion birds, respectively; figures that are similar to Banks' (1979) estimate of 10 billion birds.

But what about Alaska and Canada? Given the uncertainty about bird population densities in Alaska and Canada

All of these calculations suggest that there are far more individual birds out there than people realize. In round figures, it's probably safe to talk about minimum breeding populations on the order of 10 billion birds and minimum fall populations on the order of 20 billion birds, in North America north of Mexico. More realistically, perhaps, North America may support something on the order of 10-15 billion birds in spring and 20-30 billion in fall.

Comparative Estimates

Fisher (1940) calculated a density of 1,350 breeding birds/square mile in Great Britain, or an overall total of about 64 million birds. Using figures similar to Fisher's, Moreau (1972) estimated that 5 trillion(!) songbirds were present each autumn in western Eurasia.

Literature Cited

Aldrich, J. W. (Chair), R. C. Banks, T. J. Cade, W. A. Calder, F. G. Cooch, S. T. Emlen, G. A. Greenwell, T. R. Howell, J. P. Hubbard, D. W. Johnston, R. F. Johnston, and L. R. Mewaldt. 1975. Report of the American Ornithologists' Union ad hoc Committee on Scientific and Educational Use of Wild Birds. Auk 92(3, Suppl.): 1A-27A.

Banks, R. C. 1979. Human-related mortality of birds in the United States. U.S. Fish Wildl. Serv. Spec. Sci. Rep.--Wildl. 215, 16 pp.

Fisher, J. 1940. Watching birds. Penguin Books, Harmondsworth, Endland.

McAtee, W. L. 1931. Local bird refuges. U.S. Dep. Agric. Farmer's Bull. 1644.

Moreau, R. E. 1972. The Palaearctic-African bird migration systems. Academic Press, New York.

Peterson, R. T. 1948. Birds over America. Dodd, Mead, and Company, New York.

Terres, J. K. 1980. The Audubon Society encyclopedia of North American birds. Alfred A. Knopf, New York.


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