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First published online October 21, 2004
Journal of Experimental Biology 207, 4121-4133 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01230

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Sound localization in a small passerine bird: discrimination of azimuth as a function of head orientation and sound frequency

Brian S. Nelson^1,* and Roderick A. Suthers^1,2

¹ Department of Biology, Indiana University, 1001 E. 3rd Street, Bloomington, IN 47405, USA
² Medical Sciences, Indiana University, Bloomington, IN 47405, USA

View larger version (20K): [in a new window]   Fig. 1. Illustrations of our field (A) and laboratory (B) sound-localization experiments. In the field (A; Experiment I), flight error (FE) was calculated for each trial as the mean angle subtending the `distance' between each perch location, and the axis between the starting perch and the playback speaker. Perch error (PE) was calculated as the angle subtending the `distance' between each perch location. PE describes how perches are distributed in Florida scrub habitat and we use FE (PE/2) as our best estimate for how towhees resolve azimuth the field. In the laboratory (B; Experiment II), we employed a two-alternative forced-choice (2AFC) task in which subjects were required to discriminate between two horizontally apposed speaker positions and fly to perches associated with each speaker. To gauge performance as a function of speaker separation angle, we calculated the percentage of trials in which subjects were able to fly to the perch associated with the speaker that played the sound stimulus.

View larger version (20K): [in a new window]   Fig. 2. Illustration of the two-alternative forced-choice (2AFC) experiment used to measure head orientations when subjects were required to discriminate between two closely spaced LEDs (1.6°; see text).

View larger version (22K): [in a new window]   Fig. 3. Towhees resolve azimuth well in the field and perform nearly as well in the laboratory. Thick gray line denotes cumulative distribution of flight error observed in field experiments (Fig. 1A; N=298). Thin gray line denotes cumulative distribution of flight error after subtracting error that can be attributed to perch distribution in Florida scrub habitat (see text). Closed symbols denote performance observed in our auditory two-alternative forced-choice (2AFC) task as a function of speaker separation angle (noise 2–5 kHz; N>98; Fig. 1B). Data for each subject tested in this task are plotted separately. Open symbols denote performance observed as a function of speaker separation angle in previous 2AFC tasks conducted with other small birds (Klump et al., 1986; Park and Dooling, 1991). Asterisk denotes performance estimated for the barn owl, Tyto alba, using several methods deemed comparable with a 2AFC task (Bala and Takahashi, 2000).

View larger version (33K): [in a new window]   Fig. 4. Normalized histograms of head orientations (deg.) used by subjects when listening to stimuli presented in our auditory two-alternative forced-choice (2AFC) task (Fig. 1B; 3° histogram bins). Frequency of occurrence is represented by the radial axis and samples sizes for each subject are listed below the graph. Distributions are calculated for auditory stimuli presented from speaker separation angles <15° (<30° for subject 404).

View larger version (55K): [in a new window]   Fig. 5. Images of subject 392 captured in our auditory two-alternative forced-choice (2AFC) experiment demonstrating how this subject tended to orient his beak to the right (45°) regardless of whether his body was oriented forward (A and B) or backward (C and D) on the starting perch. In all four images the left and right speakers were located beyond the top edges of the images. Head orientations obtained for subjects 325, 000 and 404 also occurred independently of body orientation.

View larger version (31K): [in a new window]   Fig. 6. Performance varies with head orientation and with which speaker presented the sound stimulus. (A) Mean percentage of trials in which subjects 392, 000, 325 were able to fly to the correct response perch. (B–E) Performance levels observed for each subject as a function of head orientation and which speaker presented the sound stimulus. Solid lines with no symbols represent performance levels observed when stimuli were played from either speaker. Lower traces in B–E depict sample sizes without respect to which speaker presented the sound stimulus. Dashed lines indicate 65% and 75% correct performance levels.

View larger version (29K): [in a new window]   Fig. 7. Performance varies with sound frequency (2–5 kHz) and which speaker played the sound stimulus. (A) Average percentage of trials in which subjects 392, 000 and 325 were able to fly to the correct response perch when tested using a 9° or 10° speaker separation angle. (B–E) Performance levels observed for each subject as a function of sound frequency (392 at 10°; 000 at 9°; 325 at 10°; and 404 at 20°; N>100). Dashed lines indicate performance levels observed across trials in which subjects were played noise stimuli (2–5 kHz, N>200) mixed in with tones. Arrows in B–D point to sound frequencies for which subjects did not reach a 65% correct performance level, regardless of which speaker played the sound stimulus.

View larger version (19K): [in a new window]   Fig. 8. Individual subjects tended to favor different head orientations in our auditory and visual two-alternative forced-choice (2AFC) tasks, however each subject oriented similarly in both tasks (see text). Normalized distributions of head orientations used by subjects 325 (A), 392 (B) and 000 (C) in our auditory (open circles) and visual (closed circles) 2AFC discrimination tasks. Plotted above each distribution is the proportion of trials in which subjects flew to the correct response perch in the visual task. Distributions for the auditory task (open circles) are the same as those shown in Fig. 6.