Detection probability of Golden-winged Warblers during point counts with and without playback recordings

Abstract

ABSTRACT Use of point-count data to estimate population sizes of North American landbirds may be challenged by limitations on detection probability of particular species, thereby requiring correction factors to ensure accurate estimates. We estimated detection probability of Golden-winged Warblers (Vermivora chrysoptera) during 3-min point-count surveys conducted both with and without use of playback recordings in a mixed shrubland-forest habitat (clearcut area) and a 60-m wide electric transmission line right-of-way (ROW) in central Pennsylvania from 20 May to 17 June 2002–2003. In addition, we assessed the value of playback with respect to response rates of warblers and distance within which warblers approached the observer. Without playback, detection probability was approximately 23% in the clearcut area and 61% in the ROW. Use of playback resulted in 7% and 19% net increases in probability at the clearcut area and the ROW, respectively; proportional increase was approximately 30% for both habitats. Warblers responded to playback 68% of the time, but response rate was greater within 100 m (72%) than beyond (53%). Most responses (85%) included approach of the warbler toward the observer, and most individuals approached within 10 m. We conclude that 3-min point counts with playback do not yield detection probabilities sufficient to estimate population size of Golden-winged Warblers without use of correction factors. Furthermore, detection probability in mixed shrubland-forest habitats can be much lower than in linear habitats such as utility ROWs. Efforts to estimate population size of Golden-winged Warblers from data of the North American Breeding Bird Survey should recognize that habitat structure has much influence on detection probability as it relates to distance at which an observer can hear (or see) warblers. Accordingly, we recommend that such efforts incorporate a maximum detection distance of 100–150 m in mixed shrubland-forest habitats.