Cooperative Fish and Wildlife Research Units Program: Vermont
Education, Research and Technical Assistance for Managing Our Natural Resources

The probability of detecting an individual or species is an important parameter in studies using mark-recapture and occupancy models to estimate population sizes and occurrence. Because low detection probabilities result in biased estimators and decreased precision, biologists seek methods that maximize detection probability. We evaluated whether we could increase detections of bird species by playing a tape of Black-capped Chickadee (Poecile atricapillus) mobbing calls during point counts. We conducted trios of 10-minute counts (two pre-playback and a playback) at 684 stations throughout Vermont, in forested, agricultural/grassland, and developed habitats. For each of 73 species detected during the surveys, we used occupancy modeling and information-theoretic model selection and averaging methods to evaluate whether detection probabilities varied due to playback or habitat type. Models containing a playback effect accounted for over 90% of the Akaike weights for 41 species. With 15 of these species, habitat effects also accounted for over 90% of the Akaike weights. The playback increased estimated detection probability in all habitats for 14 species, decreased estimated detection probability for 20 species, and had an estimated effect that varied by habitat for 7 species (many species with habitat effects simply had differing magnitudes of the effect dependent on habitat). Smaller resident species were detected more often during tape playbacks, but responses were highly variable for most species and the responses did not appear to follow a taxonomic pattern. We encourage researchers to evaluate their list of target species carefully before deciding to use mobbing playbacks to enhance response rates; in many situations mobbing tapes will not enhance detections and may complicate the interpretation of model parameters.