Avian dependence on sound pressure level as an auditory distance cue

Sound pressure level (SPL) has received little attention as a distance cue or signal for communication because of the methodological difficulty of det ermining source SPL from free-ranging signallers and because SPL is presume d to be unreliable as a distance cue. Eastern towhees, Pipilo erythrophthal mus (Emberizidae, Passeriformes), in south-central Florida give a simple ca ll during territorial interactions. I obtained measurements of call-source SPL with a calibrated microphone positioned 100 +/- 10 cm from caged male e astern towhees. Measurements of source SPL were highly variable, but much o f this variation can be predicted from measurements of call duration or cal l frequency variables (spectrotemporal variables). Male towhees accurately perceived the distance of a speaker after it played synthetic calls that ma tched the amplitude and structure of natural 84-dB and 78-dB call types. Su bjects flew further in response to an attenuated (- 6 or - 12 dB) version o f an otherwise identical 84-dB call and flew shorter in response to an ampl ified (+6 dB) version of this same call. Towhees misjudged speaker distance in approximately half of the trials that included a discrepancy (- 6, - 12 or +6 dB SPL) between playback source SPL and predicted spectrotemporal va riables. These distance errors suggest that towhees assess auditory distanc e partly from the difference between perceived SPL and source SPL, determin ed from spectrotemporal variables. (C) 2000 The Association for the Study o f Animal Behaviour.