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Conspecific density biases passive auditory surveys

Abstract Surveys of vocalizing individuals are useful in detecting widely dispersed species that are hard to locate visually. We determined whether conspecific density affects spontaneous vocalizations of territorial Eagle Owls (Bubo bubo) and thus creates a bias in passive auditory surveys. We mainly listened to dusk call displays of 17 males breeding in two areas of high and low density from the beginning of call activity in October to the start of egg-laying in February. We detected differences in call behavior between males breeding in high density vs. low density. The nearest neighbor distance between breeding males strongly and positively affected call duration. Low density caused low detectability of male Eagle Owls, and later timing of the first call after sunset. Failing to incorporate conspecific density and nearest neighbor distances in surveys of territorial species may reduce the accuracy of population estimates.