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Aerial surveys are widely used to estimate abundance for various animals, including wetland (Conroy et al. 1988), oceanic (Finley et al. 1987), and terrestrial (Bear et al. 1989) species. A variety of sampling plans and estimators have been used in surveys (see Seber 1982, 1986, 1992). Statistical sampling theory may suggest which of various sampling plans or estimators are appropriate under certain circumstances, but there may be restrictions such as using large sample sizes. Rules of thumb given for sample sizes (Moore and McCabe 1993:510) may not be appropriate for the skewed distributions often characteristic of wild animal populations. Known populations can be used in evaluating sampling plans and estimators.

Despite their importance and wide use, sampling plans and estimators have rarely been tested on wild animal populations for which actual abundance and distribution of individuals were known (Davis and Winstead 1980:244, Seber 1982:561); exceptions include a survey of bison (Bos bison) on an island (Wolfe and Kimball 1989) and a mule deer (Odocoileus hemionus) count within enclosed pastures (White et al. 1989). Most evaluations involved natural populations of unknown abundance and distribution (Bergerud and Manuel 1969, Redmond et al. 1981, Firchow et al. 1990) or simulated populations of known abundance and distribution (Zarnoch 1976, Caughley 1977). Even less is known about the performance of sampling plans and estimators when sampling animals tend to cluster. We obtained aerial counts and locations of pronghorn for 2 areas (1,242 and 2,387 km²) in North Dakota in 1979, 1986, and 1987. Our objective was to evaluate several sampling plans and estimators on populations of known sizes of a species that is spatially clustered.

We thank M. D. Schwartz and R. M. Woodle for technical assistance, and E. Forgaard, J. W. Wyckoff, and the University of North Dakota, Department of Geography, for habitat maps. We are grateful to J. E. Austin, R. R. Koford, W. E. Newton, J. R. Sauer, D. J. Twedt, and 4 anonymous referees for comments on earlier manuscript drafts. Aerial surveys were partially funded by North Dakota Pittman-Robertson Project W-67-R.