BIASED DETECTION OF BIRD VOCALIZATIONS AFFECTS COMPARISONS OF BIRD ABUNDANCE AMONG FORESTED HABITATS

Community studies of birds often rely on abundance estimates that are obtained from counts of bird vocalizations, yet vocalizations are not equally detectable in all habitats. I broadcast vocalizations for nine bird species to evaluate biases in detection of bird vocalizations am ong four forested habitats (young, mature, and old aspen dominated for ests, and white spruce dominated forests), and in relation to height o f the broadcast, whether the broadcast occurred before or after leaf f ormation, and the frequency of the broadcast vocalization. Virtually a ll of the broadcast vocalizations were detected at 50 m from the speak er. However, at 100 m from the speaker, 27% of the broadcast vocalizat ions were not detected and detection was highest in white spruce fores t, lowest in young aspen forest, and intermediate in mature and old as pen forests. Detection of broadcasts was negatively related to the min imum frequency of the vocalization, higher for broadcasts from the can opy than for broadcasts from the shrub layer, and higher for broadcast s before than after leaf formation. I reanalyzed abundance data that w ere obtained from a study involving point counts of wild birds in youn g and old aspen-dominated forest. Biases among habitats in the detecti on of vocalizations had moderate influence on the resulting measures o f habitat preferences for birds. I suggest that if a detection distanc e of more than 50 m is used for bird censuses within forested habitats , then comparisons among forest types should be interpreted cautiously unless the researchers demonstrate that biased detection of vocalizat ions does not affect their conclusions.