The parrots of all four species (except for two grey parrot individuals that therefore did not take part in the subsequent tests) selectively chose tokens associated with more preferred foods first, handing the provided tokens back in a predictable order according to preference. These findings parallel earlier findings in primates21,24 and suggest that the parrots indeed attributed value to the tokens. When provided with a choice between a food or a token that could be exchanged for more preferred food, all four species inhibited their impulsive reactions and selected the token significantly more often than chance, thus maximising their payoff. They did so regardless of the value of the immediate gain, i.e., low (condition 1) or medium (condition 2). In fact, their performance in condition 1 was comparable to that of chimpanzees (with three out of three chimpanzees selecting the token over the food to increase their yield)27 or even slightly better than that of capuchin monkeys (with two out of four monkeys choosing economically)28 or Goffin cockatoos (five out of 13 birds selecting the tool that led to a higher payoff)29. Note that all four capuchin monkeys in the Judge and Essler study28 failed to maximise their payoff when faced with the choice between medium-value food and high-value token (condition 2 in the present study), although one individual succeeded in a later re-test. Taken together, our results strengthen previous findings of high level of inhibitory control in parrots in delayed gratification tasks16,17, despite their apparently poor motor self-regulation performance when tested in a detour reaching task34,35.

More critically, when facing the choice between food and a token through which they could obtain food of lesser value (condition 4), all birds of all four species (except for one blue-throated macaw individual) selected the immediate food reward over the token, even if it was not the highly preferred food. These findings are in line with the previous results found in primates and suggest that the parrots did not merely learn a “select a token over food” rule. In contrast to the study on chimpanzees, which had to choose between the low-value token and high-value food27, the parrots and previously tested capuchins monkey28 faced an arguably more difficult choice in this condition, because the discrepancy in value between the two options was less pronounced (parrots and capuchins: MF vs LT, compared to chimpanzees: HF vs LT). When a medium-value token was offered as the alternative to the low-value dry corn (condition 3) however, some individuals of the blue-throated and the blue-headed macaws had more difficulty to decide economically and picked randomly. The higher number of mistakes in this condition suggests that either the high token had an intrinsic value for them, or the difference between the immediate and the future gain was not sufficiently pronounced to overcome their desire for immediate satisfaction.

The remaining two controls presented situations where the immediate and future yield was of the same quality to test whether the subjects would avoid the unnecessary effort and extra waiting time by selecting the immediate food reward. The majority of the grey parrots performed equally poor in both conditions and unnecessarily chose the token suggesting that they merely learnt a “select a token over food” rule. However, the outstanding performance of the grey parrots in the condition 3 MT (MF vs LT), where choosing the token would result in obtaining a food of lesser value contradicts this assumption. Similarly contradictorily, the blue-headed macaws performed well when faced with a choice between token and food of the medium value (condition 5), but failed to optimise their choices when faced with the most preferred food (a piece of walnut) and its associated token (condition 6). This may imply that the high-value token had acquired an intrinsic value for them, and the same might be true for most of the grey parrots. From a comparative perspective, the large macaws performed comparably to chimpanzees, which in the Beran and Evans (2012) study always selected food over lexigram tokens representing the same food27. Overall the parrots’ performance in these two conditions compared to that of Goffin cockatoos tested in a comparable situation. Five of 13 Goffin cockatoos selected their most preferred food over a tool that could be used to obtain the same food29 (an analogue to condition 6 in the present study).

In summary, on a group level, the great green macaws were the only species in which most of the individuals performed significantly above chance throughout all six test conditions, suggesting that they successfully weighed up between the two options to make the most advantageous choice. These findings are in line with previous studies showing that cockatoos, a distantly related psittaciforme, also decide economically. Considering that the blue-throated macaws performed well overall but showed a weaker performance only when the token offered was of fairly low value compared to the immediate reward (condition 3), we assume that motivational issues might have confounded the results. For them, the difference between the immediately available corn and the potential sunflower seed, which they could have traded the medium-value token for, might not have been sufficiently pronounced to justify the extra effort/additional waiting time of another exchange. In contrast to the larger macaws, the blue-headed macaws appeared to generally prefer the high-value token over the immediate food, even if it was of the same value. We therefore cannot exclude that they might have followed the rule “always take the high token, otherwise take the food”. Alternatively, however, their economic decision-making ability might have been confounded because they appeared affected by the intrinsic value of the high-value token. The majority of the African grey parrots failed to optimise their choice only when the token was of the same value than the immediately available food, i.e. in conditions 5 and 6. Here they often chose the token despite the unnecessary effort. One possible reason for their uneconomic behaviour may be that they found interacting with the higher value tokens as self-rewarding and thus might not have perceived trading them as a notable effort. This is not unlikely, given that some parrot species have strong tendencies to engage in object play36. It would be interesting if future studies tested whether increasing the costliness of the token exchange (e.g., prolonging the waiting time before exchange or adding effort) would change their behaviour.

In our study, the two larger macaw species stood out in the number of individuals that chose optimally in all six test conditions. Specifically, they outperformed the others in control condition 6, thus suggesting that they were more likely to be deciding economically compared to the smaller blue-headed macaws and the grey parrots, who may have used a simpler rule to always choose the high token. A species’ feeding ecology has often been suggested as one of the selective forces affecting inhibitory control and evolution of economic decision-making35,37,38. The great green macaws and the blue-throated macaws are supposedly feeding specialists with pronounced dependence on nuts and fruits, thus ephemeral food sources31,39, while the blue-headed macaws’ and African grey parrots’ diets have a broader spectrum among our tested species32. However, ecological explanations for this difference are highly speculative and the exact cognitive demands of the species’ feeding ecology are a matter of ongoing dispute and can be viewed in diverse ways35,38,40. For example, one might consider the ephemeral nature of the larger macaws’ main food sources less demanding in terms of the need for pay-off maximisation since the birds should always exploit any limited food source they encounter38,40. However, our data stand in contrast with this assumption. On the other hand, one may argue that economic decision making might play a greater role in feeding specialists than more generalist and thus opportunistic species, as they may have to weigh up when to leave one food source in order to reach the next in time, in which case our data will be in line with this expectation. Whatever argumentation one might be willing to follow, one should keep in mind that the foraging behaviour of most wild parrots is very difficult to assess and any correlations between economic decision-making and feeding ecology should be considered with precaution until more data become available.

Another factor that ought to be considered when construing the species’ differences in the performance may be a minor methodological difference in the order of test conditions. The African grey parrots, one blue-throated macaw (Mowgli) and one great green macaw (Shouty) were administered trials of the six different conditions within the same session, whereas all the other birds received the trials of conditions 1, 2, 4 and 5 and the two control conditions (3 and 6) in separate sessions. However, no pattern was found suggesting that the species’ performance was affected by having conditions administered in two different sessions. For instance, the blue-headed macaws, which can be considered as the worst performing species, mostly failed in the two control conditions (3 and 6) that were administered separately from the other four conditions, while the great green macaws performed equally well in the sessions with the four conditions (1, 2, 4 and 5) and in the sessions with the two additional controls (3 and 6). The blue-throated macaws failed in the control condition 3, but not in the condition 6, even though both were administered separately from the other four conditions. Also, among the birds that were administered trials of the six different conditions within the same session, no pattern suggesting that the testing order of conditions had played a role was observed. While the grey parrots performed particularly poorly in conditions 5 and 6, the blue-throated macaw (Mowgli) failed in conditions 3 and 5 and the great green macaw (Shouty) performed exceptionally well in all six conditions.

In summary, the parrots in the present study inhibited their impulsive reactions if it led to an increased payoff. Particularly, the performance of the two larger macaws was equally good or even better than the performance of primates and cockatoos in comparable conditions, and most importantly, in the critical controls, suggesting that they have the capacity to decide economically and optimise their behaviour. The blue-headed macaws and African grey parrots inhibited their impulsive reactions by choosing tokens over food but failed to choose economically in some of the control conditions. We suggest that their capacity to choose economically might have been affected by certain confounding factors as outlined above that ought to be carefully considered by future token exchange studies.