Use of Presence-Absence to Measure Changes in Avian Density

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Abstract
We evaluated the use of frequency, or presence-absence, data to estimate change in bird density as measured by change in number of individuals recorded. We used data from the Breeding Bird Survey (BBS) and compared the number of individuals recorded/route and the number of stations (out of 50)/route at which the species was recorded. The mean correlation coefficient between number of individuals and number of stations, for 190 species, was 0.997, indicating that general trends estimated using the 2 measures are usually similar. We illustrate this similarity with graphs showing change in number of individuals/route and in number of stations/route, with mean values superimposed. Stations usually changed less than individuals, especially when several individuals commonly occurred at a single station. In such cases, trends estimated from station data were often 40% lower than trends estimated from counts of individuals. Results were quite variable among the data sets that seems to preclude any simple method of adjusting station data. Stations/route is an acceptable surrogate for individuals/route for detecting a population change but will usually be unacceptable for estimating the magnitude of change in individuals. Other uses of BBS data exist for which stations/route are of no use. Counting individuals of each species, rather than simply recording presence of each species, is thus advisable unless recording only species has substantial advantages (e.g., attracting more observers and thereby obtaining a larger data set).