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Breeding Population Inventories and Measures of Recruitment

IV. Sampling

If estimates of breeding population or recruitment pertain to a single site where complete enumeration of breeding population is possible, sampling error may not be a problem. Intentionally or unintentionally, however, results from field studies on local study areas are often extrapolated to a much larger geographic area. Breeding population and recruitment vary greatly among study sites because of variation in weather, habitat, and predator populations (Greenwood et al. 1987). A number of study sites, therefore, are required to obtain reliable estimates applicable to a large geographic area. The among-study-site variation in recruitment rate is usually so large that obtaining a sufficient number of ground plots to gain a reasonably precise regional estimate would require an army of field biologists, but ground plots may be practical for estimating breeding population (Brewster et al. 1976). For large areas, estimates of breeding population or broods are usually based on aerial surveys, which permit a large number of sample plots or transects. Jolly (1969a) discussed sampling methods for aerial censuses. Much of the discussion and methodology presented in that paper is equally applicable to sample designs for ground counts.

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