We calculated the mean taste-rejection probability for defended and undefended prey across all trials. As expected, defended prey were rejected more often than undefended prey (F1,33 = 55.79, p < 0.01; ). The rejection rates of both defended and undefended prey were positively correlated with the mean defence level of the population (F1,33 = 6.601, p < 0.01), with no significant interaction (F1,33 = 0.149, p > 0.05). Defended prey in the moderately defended group were rejected significantly more than defended prey in the mildly defended group (t-test: t = 3.23, p < 0.05, d.f. = 23), but there was no significant difference in the rejection of mildly and moderately defended prey in the mixed defence group (paired t-test: t = 0.29, p > 0.05, d.f. = 10; ).
The mean (±s.e.) rejection probability of undefended (white bars), mildly defended (grey bars) and moderately defended prey (black bars) in the three experimental treatments.
Taste-rejection behaviour on each prey type also changed across our six trials (). We found significant interactions between trial and group on taste rejection for all prey types (GEE; undefended: Wald χ2 = 44.88, p < 0.01; mildly defended: Wald χ2 = 13.82, p < 0.05; moderately defended: Wald χ2 = 13.90, p < 0.05; ). We subsequently compared rejection probabilities between trials 1 and 6 using paired t-tests. For undefended prey, we found a near significant decrease in rejection rates in the mildly defended group (t = 2.05, p = 0.06), an increase in the moderately defended group (t = 5.56, p < 0.01), but no change in the mixed defence group (t = 0.21, p > 0.05). For the mildly defended prey, rejection probability decreased in the mildly defended group (t = 3.32, p < 0.01), but there was no change in the mixed defence group (t = 0.46, p > 0.05). We also found that the rejection of moderately defended prey increased in the moderately defended group (t = 3.23, p < 0.01), with no change in the mixed defence group t = 0.14, p > 0.05). Overall, chicks in the mildly and moderately defended groups were changing their rejection behaviour towards defended and undefended prey during the experiment, while there were no changes in the mixed defence group.
The mean (±s.e.) rejection probability across trials for (a) undefended, (b) mildly defended and (c) moderately defended prey in the mildly defended (squares), mixed defence (triangles) and moderately defended (circles) groups.
The differences in rejection behaviour were based upon the taste of the prey and not on any visual differences. We compared the probability of attack for each prey type to what would be expected at random (i.e. 0.4, as 16 of 40 crumbs were attacked in total). This was not significantly different from random for any prey type in any group (one-sample t-tests: t < 2.01, p > 0.05 for all comparisons).