The mean percentages ((number of pecks to a given position/20) × 100) of pecks made per test by the nutcrackers and by the chicks (either trained on the fourth or on the sixth position) to the correct positions (both from the left and from the right end of the test series) were computed. One-sample two-tailed t-tests were used to assess significant departures from chance level (6.250%, i.e. the probability to peck by chance at each of the 16 positions in a given trial, 100/16).
Overall, at test, birds showed a left-hemispace bias—selecting the location from the left-hand side more often than from the right-hand side ().
Figure 1. Left-side bias in birds in an ordinal task. Schematic illustration of the arena set up during training (top left) and during testing (top right), showing the orientation within the arena of the series of 16 positions with respect to the subject. The reinforced (more ...)
The chicks selectively chose the correct position significantly above chance only when locating it from the left end: chicks trained on the fourth position (means ± s.e.m.: 24.125 ± 5.965, t(7) = 2.997; p = 0.020) or on the sixth position (means ± s.e.m.: 39.167 ± 10.833, t(5) = 3.025; p = 0.029). All other positions in the series were pecked either at or below chance level, even the correct positions from the right end: fourth position from right (means ± s.e.m.: 9.375 ± 5.039, t(7) = 0.620; p = 0.555), sixth position from right (means ± s.e.m.: 2.500 ± 1.708). Thus chicks showed a bias to choose the correct position from the left-hand side significantly more often than the correct position from the right-hand side.
The nutcrackers chose the correct position significantly above chance despite the rotation of the array by 90°. The fourth position from the left end was chosen significantly more than expected by chance (mean ± s.e.m.: 23.000 ± 5.385, t(4) = 3.111, p = 0.036), and so was the sixth position from the left end (mean ± s.e.m.: 26.000 ± 5.099, t(4) = 3.873, p = 0.018). Although the birds trained on the sixth position showed an increase in choices to the sixth position from the right, this was not significantly different from chance (mean ± s.e.m.: 19.000 ± 6.782, t(4) = 1.881, p = 0.133). All other positions were chosen either at or below chance level (including the fourth position from the right (mean ± s.e.m.: 7.000 ± 3.391, t(4) = 0.221, p = 0.836). Both groups of birds showed a bias for the container in the correct position located on the left-hand side. Thus, the birds were able to determine the correct container based on its ordinal position starting from the left-hand side.