Population Densities of Birds Derived from Transect Counts

GoOD estimates of absolute population density as distinct from indices of relative abundance have been virtually unavailable for nonflocking land birds except in the breeding season when singing males, representing mated pairs, restrict themselves to more or less fixed territories where they or their nests can be counted. The lack of efficient and reasonably accurate census methods applicable at any season has seriously hampered the progress of quantitative studies of avian population ecology. This paper, after reviewing the potentialities and limitations of currently available methods, describes a new method that is, 1) applicable at all seasons, 2) more efficient in terms of area covered per unit of effort than the nest or territory count methods, and 3) comparable in accuracy. The method uses the lateral distribution pattern of all detection points for each species to derive coefficients of detectability with which trail counts may be converted directly to density values in units of birds per 100 acres. The method was developed over a period of 3 years while the author was gathering data on the ecological distribution of birds in mesquite grasslands in southern Texas, pine forests in Florida and the Bahamas, and mixed woodlands in Wisconsin and Michigan.