Effects of experimental playbacks on availability for detection during point counts

Point counts are the most commonly used technique for surveying passerines during the breeding season. Several methods for estimating probabilities of detection during point count surveys have been developed. These methods have focused primarily on accounting for the influence of environmental factors (e.g., weather and noise) on detectability, however, the probability that birds are available for detection (e.g., sings or moves) during point counts has received less attention. We used sequential point counts to determine the effect of playback of the mobbing calls of Black-capped Chickadees (Poecile atricapillus) and the flight calls of Red-tailed Hawks (Buteo jamaicensis) on availability for detection (e.g., singing or moving) during point-count surveys. We conducted 180 point counts over a 2-yr period in central - east central Minnesota to evaluate the possible effect of playbacks on observed density, overall species richness, minute of first detection, and distance of first detection. We also used removal models to quantify the magnitude of changes in detectability and direction of response to playbacks for 10 focal species. Playback of the mobbing calls of Black-capped Chickadees increased observed density and decreased the average distance of detection and time of first detection, whereas playback of the flight calls of a Red-tailed Hawk resulted in a decrease in observed density and species richness, and an increased time of first detection. Playback treatment was a covariate in all best performing models for the 10 species analyzed, but the magnitude and direction of response to playbacks were species specific. The importance of playback type in detectability models indicates that the calls of heterospecifics can influence species availability for detection. As such, researchers using playback methods should seek to quantify species-specific responses in detection probability and consider how component detection probabilities could influence survey outcomes. Los conteos de punto son la técnica más común para sondear o detectar pasennos durante la época de reproducción. Se han desarrollado varios métodos para hacer un estimado de la probabilidad de detección durante conteos de punto. Estos métodos se han enfocado, primeramente, para tomar en cuenta la influencia de factores ambientales (ej. ruido o las condiciones del tiempo), en la detectabilidad. Sin embargo, la probabilidad de que los pájaros estén disponibles para ser detectados (ej. cantos o mo