Test of partners in flight effective detection distance for Cerulean Warbler / Evaluación de distancia efectiva de detección para la Reinita Cerúlea (Dendroica cerulea) seleccionada por Compañeros En Vuelo
Estimation of population sizes of North American avian species has been attempted in the North American Landbird Conservation Plan. Such estimated numbers have considerable conservation value as starting points to estimate extinction probability, as was done for Cerulean Warbler (Dendroica cerulea) during the U.S. Fish and Wildlife Service evaluation of the petition to list the species as Threatened. Population estimates presented in the Flight Plan reflect assumptions applied to counts reported by observers on Breeding Bird Survey routes. One of these assumptions is the assignment of species to effective detection distance radii. We chose to test the assumption that effective detection distance of 125 m for Cerulean Warbler was an adequate value in bottomland hardwood and other forests in the species’ breeding range. We randomly selected roadside and off-road locations, visited each multiple times with multiple observers, and used hand-held Global Positioning System units to measure the distance between count station and birds detected aurally. We used multiple covariate distance sampling to analyze these data in Program Distance. Our best estimate of effective detection distance is 94 m (95% CI 88–101 m), significantly lower than 125 m. Consequently, the total population estimate of Cerulean Warbler in the North American Landbird Conservation Plan, 560 000, should be revised to approximately 875 000; assuming all other factors involved in the calculation of total population remain equal.