FIG. 2.

(Color online) (a) Representative behavioral results from two tracking sessions in budgerigar B47. The formant frequency of the test stimulus was varied according to a two-down, one-up, adaptive tracking method. Each trace plots the absolute value of the formant-frequency difference between the target stimulus and the standard stimulus versus target trial number. Discrimination threshold was calculated as the mean y-value of the last eight reversal points (circles). Thresholds for the sessions shown, for F1 discrimination with F2 present (experiment II), were 23.50 ± 3.21 Hz (black) and 20.8 ± 2.05 Hz (red; means ±SD). (b) Representative behavioral results showing threshold variation across test sessions. Thresholds are for F1 discrimination with F2 present, in B47. × markers near the top of the plot indicate non-tracking sessions. Sessions for this condition were conducted in three cycles, between which other stimulus conditions were tested (36 sessions between test cycles one and two; 120 sessions between test cycles two and three). A new vertical axis is drawn at the start of each new test cycle. For each cycle, threshold was calculated as the mean of the last six session thresholds (filled symbols). The third testing cycle was conducted due to a significant decrease in threshold between the first (28.67 ± 4.05 Hz) and second cycles (21.46 ± 3.04; t₁₀ = −3.486, P = 0.006). The threshold for this condition (20.00 ± 2.99 Hz) was calculated as the mean of the last two test-cycle thresholds, which were not significantly different from each other (t₁₀ = 1.872, P = 0.091).