The efficiency of territory mapping, point-based censusing, and point-counting methods in censusing and monitoring a bird species with long-range acoustic communication - the Corncrake Crex crex

Capsule A comparison of various methods of counting a Corncrake population revealed that the accuracy of the methods depends on population density, which could have important ramifications for a monitoring scheme. Aims We examined the accuracy of three counting methods - territory mapping, point-based censusing, and point counting - in evaluating the population size and spatial distribution of the Corncrake, a species that uses long-range acoustic communication. Methods We performed single-visit counts within ten study plots (1 km(2)). Each study plot was censused in a single night using all counting methods. With territory mapping, we approached each calling male and recorded his position with a Global Positioning System (GPS) receiver. In point-based censusing, we estimated the distance and direction to calling males from five points in each study plot, while with the point-counting method, we only recorded the number of males within each of four distance categories, which estimated the distance from the same five counting points in each study plot. To estimate males' density within the study plot using the last method, we used only the data obtained from the central point of each study plot (middle-point counting approach). Results We found significant differences among the three methods in the number and distribution of males counted within the study plots. The middle-point counting approach consistently underestimated population size regardless of the number of males present, while the point-based census approach overestimated population size when a small number of males were present within a study plot, but underestimated it when males were numerous. Conclusions Our study showed that the accuracy of counting methods may depend on bird density. Moreover, point-based censusing may prove quite inaccurate in pinpointing the locations of calling males. When censusing species with long-range acoustic communication, it is challenging to estimate distance to calling birds, which means that the standard point-counting method should be applied carefully.