Peer review of: Kea show three signatures of domain-general statistical inference (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
The present study presented six kea individuals with tasks requiring statistical inferences from 
population to sample, using a similar methodology as has been applied in previous studies with human 
infants, nonhuman great apes and monkeys. In a series of three experiments, subjects seemingly not 
only used information about the proportional composition of populations in order to predict sampling 
outcomes (Experiment 1), but also seemed to consider information about physical (Experiment 2) and 
social constraints (Experiment 3). The authors conclude that kea are capable of true statistical 
inference- a capacity that has previously been found in only few nonhuman species (mainly primates). 
Moreover, the authors claim that the birds´ capacities even exceed those of New and Old world 
monkeys: While capuchin monkeys and long-tailed macaques failed crucial test conditions controlling 
for the use of absolute quantity heuristics in previous studies, kea seemingly succeeded in similar 
controls in the present study. Lastly, a main conclusion of the paper is that kea can integrate 
information from different cognitive domains (physical and social) into their statistical inference and 
are therefore capable of domain-general thought. This conclusion leads the authors to speculate that 
domain-general statistical inference may have evolved in a convergent manner in distantly related 
species. 
 
The topic of the present study is highly relevant, up to date, and interesting for a broad readership. So 
far, the study of intuitive statistics has mainly focused on (human and nonhuman) primates. However, 
to explore the evolutionary roots of this fundamental capacity, it is important to consider the 
possibility of convergent evolution and investigate this capacity in a wider range of species. Birds with 
complex cognitive abilities, such as parrots and corvids, seem promising candidates for finding 
statistical abilities outside the primate lineage. The present study, therefore, fills an important gap in 
the literature and will most likely stimulate further research in this area. The paper is well written, 
clear and concise. I particularly appreciate the authors´ effort to test not only for the “standard” 
capacity of reasoning from population to sample, but also to investigate whether birds can use 
information from different cognitive domains when drawing statistical inference. Hence, I believe that 
this is a very promising, comprehensive, and relevant study, and the authors have the data to support 
most of their claims. That being said, I have several points of criticism/suggestions for improvement 
which I feel should be addressed by the authors. 
 
 
Introduction: I think the introduction (especially the first paragraph) would benefit from more details. 
Some terms and concepts are not sufficiently explained for the readership of a multidisciplinary 
journal such as Nature communications. For instance, the authors never define what they mean with 
the term “cognitive domain”, and many readers may not be familiar with “g”. Considering the 
importance of these terms for the present study, I believe it would be wise to add more explanation in 
the beginning of the manuscript. The literature review seems fairly up to date. However, I would 
suggest adding the following important references: 
 
Page 3, line 37: Teglas et al., 2011 (one of the most convincing demonstrations of infants integrating 
physical (spatio-temporal) and statistical information). 
 
Page 4, line 48: Eckert, Call, et al., 2018 (the first study to rule out absolute quantity heuristics both 
regarding preferred and non-preferred food items in great apes); 
Placi et al., 2019 (long-tailed macaques use relative, rather than absolute frequencies in a statistical 
reasoning task with a different task format than in previous studies). 
 

Page 4, line 51: Roberts et al., 2018 (Pigeons use relative, rather than absolute quantities in a 
probabilistic reasoning task). 
 
Both Roberts et al. (2018) and Placi et al. (2019) suggest that not only great apes may be capable of 
true statistical inference -> the authors might want to rephrase the sentence on page 4, lines 47f. 
 
 
Experiment 1: My main criticism is that this study did not test a spontaneous and intuitive ability, but 
rather trained subjects to reach criteria in the different tasks, which is a crucial difference to previous 
studies on human infants, great apes, and monkeys. Hence, we do not know whether birds truly 
inferred the outcome of the sampling event, like primates did, but rather learned an association 
between one of the populations and a reward over the course of trials. In contrast to ape and monkey 
studies, the authors do not report first trial performance or test for an effect of trial number on the 
subjects´ performance in order to shed more light on this possibility. This problem is aggravated by 
the fact that the authors did not counterbalance the order of conditions- hence, learning across 
conditions is possible, and perhaps even likely, especially considering that some of the populations 
remained the same across several conditions. For instance, both in Condition 1 and 2, the unfavorable 
population contained a 20:100 mixture. After having received on average 120 trials in Condition 1, 
kea most likely knew that the 20:100 population would not lead to a rewarding token as a sample 
when it was presented to them again in Condition 2- they did not need to draw any statistical 
inference. This learning explanation is supported by the fact that birds became gradually better over 
the course of the three test conditions, a fact that would otherwise be surprising considering that task 
difficulty increased and the magnitude of difference between the two populations decreased (which 
would make statistical reasoning presumably more difficult). I understand that for birds the general 
procedure might be less intuitive/familiar than for primates, and therefore more training might be 
necessary. However, ideally this training should have been administered as a pre-training only, and 
not involve the same populations as in the actual test conditions. 
 
The authors mainly rely on the birds´ good results in Condition 2 and 3 when claiming that kea, in 
contrast to monkeys, are capable of true statistical inference based on relative rather than absolute 
quantity. However, given the methodological discrepancies between the present study (several 
hundreds of trials per condition, using the same population across conditions) and primate studies (12 
trials per condition, different populations for each condition), I think it is misleading to conclude that 
kea, but not monkeys, succeeded in statistical inference tasks in which absolute and relative 
frequencies were disentangled. 
 
I suggest that the authors re-run Condition 2 with new populations (different ratios) to verify their 
claim. If this is not possible, the discussion should acknowledge the methodological differences 
between bird and monkey studies, as well as the alternative explanation that birds may have formed 
associations between populations and rewards in Experiment 1. 
 
 
Experiment 2: For this experiment I have a question regarding the procedure, in particular what 
exactly the birds could see: Did they see that the experimenter´s hand reached only into the upper 
compartment of the jar? Or did they just see that the hand was inserted into the jar, but not into 
which compartment it reached? I think this question is crucial, because it may tell us how the birds 
represented the scene. If kea saw that the experimenter´s hand only reached the upper 
compartment, they did not necessarily have to consider both statistical and physical circumstances in 
order to solve the task; it would have been sufficient to perceive the two compartments as two 
separate populations- similar as if they would have been in two different jars. Again, this is an 
important difference to the infant studies testing for the integration of statistical and physical 

information (e.g. Teglas et al., 2007; 2011; Denison & Xu 2010; Denison et al., 2014). In these 
studies, infants inferred the effect of the physical determinant (e.g. barrier) on the sampling event 
without actually seeing the effect (because the sampling event itself was occluded). 
 
If, in the present study, kea did see the effect of the barrier (i.e. that the hand was only inserted in 
the upper container), the authors should acknowledge this difference to infant studies in their 
discussion and tune down the conclusions drawn from this experiment. Kea´s performance in this 
experiment is still impressive and a first hint that birds might consider a physical constraint when 
drawing statistical inference. But the here demonstrated abilities are clearly qualitatively different 
from those that have been demonstrated in human infants. 
 
 
Experiment 3: In this experiment I do not understand the rationale of the final experience phase 
(memory probe) with E2. Why was it beneficial to give kea extra experience with only one of the two 
experimenters before the actual test? 
The beauty of the original experiment (which was developed for chimpanzees; Eckert, Rakoczy, et al. 
2018) was that subjects were non-differentially rewarded by the two experimenters in the 
demonstration phase, before they were allowed to make a choice between them in the test phase. 
Hence, chimpanzees had no way of associating one of the two experimenters positively or negatively, 
and instead had to use their statistically acquired knowledge about the experimenters´ biases as only 
source of information. 
 
By contrast, in the present study, the authors included an additional experience phase in between the 
demonstration phase and the test phase. Crucially, in this phase the birds were given additional 
experience with only one of the two experimenters (E2), therefore weakening the non-differential 
character of the demonstration phase. In other words: The latest experience that birds had with each 
of the experimenters by the time the test started was “E1 always samples rewarding tokens”, “E2 
sometimes samples rewarding tokens and sometimes unrewarding tokens”. Hence, it is unclear 
whether birds succeeded in the test condition truly by the means of statistically acquired knowledge or 
rather by a strategy to avoid the experimenter which had given them unrewarding samples in their 
previous encounter. Again, if the authors are not able to rule out this alternative explanation, they 
should acknowledge it in the discussion and tune down their interpretations. 
 
 
Additional remarks: In my opinion, the term integration of information from different cognitive 
domains is not appropriate. What the authors could show in this study (if the above-mentioned points 
are sufficiently addressed/clarified) is that kea flexibly decided which of two sources of information 
(e.g. proportional distribution of items vs. information about a physical barrier) will determine the 
outcome of a sampling event. An integration of these two sources of information would imply that the 
influence of each factor is weighted and included in the final computation, leading to a gradual 
response (similar as in, e.g. Teglas et al., 2011). 
 
The reference list contains some formatting errors. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
Bastos & Taylor 
Kea (Nestor notabilis) show three signatures of 
domain-general statistical inference 

 
This paper aims at testing whether members of a parrot species (Kea) show “true statistical 
inference”. The general approach is to show the subjects transparent jars filled with various numbers 
of black and orange plastic items, which the birds are trained to exchange for food items, hence they 
are ‘tokens’. 
The paper presents 3 experiments with different conditions each, including keeping the number of +ve 
items constant but varying the number of -ve ones, and varying the absolute numbers as opposed to 
(or as well as) the proportions. 
I enjoyed the paper and the experiments, but had one conceptual difficulty. All the conditions and 
conclusions are presented and discussed using frequentist descriptions (for instance, in one condition 
a jar contained 63 +ve and 57 -ve tokens while the other jar the reverse numbers). In fact, it is not 
possible for any observer (even a human) to ‘count’ the tokens in a jar, since they are distributed 
randomly in a volume so that some are behind. Further, it is unreasonable to suggest without 
evidence that the birds count such high numbers. 
It is more likely that they use the ‘blackness’ and ‘orangeness’ that they can see to categorize the 
jars. That would be a useful heuristic for the probability of extracting a black or orange bar. In that 
case, the conditions can be described in terms of relative (or absolute) areas of a given colour. For 
instance, a jar with 20 +ve and 100 -ve tokens should be described as averaging 16.7% black area. 
This heuristic is perfectly correlated with changes in both absolute and relative numbers, and leads to 
using a relative assessment of probability of reward. 
I am not clear on whether this change in the description would harm the interpretations the authors 
put forward, but would like to see that discussed. With frequencies, 2/10 is very different from 20 
/100, but with areas, this is not the case, as one observes one sample of each mixed colour per jar 
rather than 12 and 120 samples respectively. 
The authors make a sharp distinction between “true statistical inference” and “heuristics”, but this 
distinction is not clear to me in the present context. The birds must do the job somehow, namely they 
must have a perceptive/psychological mechanism to translate what they see to a behavioural choice. 
They might, for instance, acquire an inhibitory trend towards orange and an appetitive one towards 
black. The balance between these two tendencies can determine choice probability. Is this a heuristic 
or a true inference? It seems that their dismissal of heuristics is in fact a dismissal of any realistic 
algorithm. As a minimum, I’d like to see a more conclusive differentiation between ‘true inferential’ 
and ‘heuristic’ decision making. 
A couple of points about the treatment of taxa: When I read the following lines “kea clearly mirrored 
the type of statistical inferences made by humans and great apes when solving these problems, rather 
than using heuristics, as monkeys do.” and “evidence that capuchins and macaques are not capable of 
true statistical inference” I was startled by the strength of confidence on what all monkeys are 
supposed to not being able to do. This is based on a few papers in a couple of monkey species. 
Affirming the inability of any taxon to do something has a history of failing and warrants caution. 
People used to say that humans were the only species using tools, and other statements that sound 
ludicrous today given advanced knowledge. Is it necessary to restate that absence of evidence is not 
evidence of absence? Comparisons between the cognitive abilities of different species is notoriously 
difficult and as a minimum requires testing a battery of different protocols, since one species may use 
a simple algorithm in one context and a complex one (true inference?) in another. Referring to a 
diverse taxon requires exploring many species within the taxon and with many protocols. Such 
sweeping statements can be misinterpreted as a bias towards overselling the ‘intelligence’ of one’s 
working species, always a bad habit. 
I was similarly surprised by reading the statement “Much work in this field over the past 15 years has 
focused mainly on corvids, which have produced their most impressive problem-solving performances 
predominantly on domain-specific, ecologically relevant tasks, such as those involving caching or tool 
use40, 41.” In the period referred to there has been a huge amount of work on parrots, including the 
seminal work on African greys by Pepperberg, as well as work comparing kea (the topic of this study) 

with corvids on tool using and non-tool using protocols, work on problem solving, social learning and 
tool use in Goffin cockatoos, work with token exchange in other parrot species, etc etc. This seems a 
very biased way to place the present study in context. 
Alex Kacelnik 
 
 
 
Reviewer #3: 
Remarks to the Author: 
This paper address an interesting question in comparative cognition about the ability of animals to 
process probabilities and make subsequent inference about the likelihood of an event to happen. The 
results presented strongly suggest (but see my concerns below) that a parrot species, the kea, not 
only can compute probabilities but also combine physical and social information to improve their 
inference. The fact that this species shares this ability with some primates results certainly from an 
independent evolutionary history that raised the intriguing question of the ecological factors favouring 
such complex cognition. The second conclusion of the paper is that keas seem to rely on true 
probabilities rather than absolute counting abilities (heuristic mechanisms) that have not been 
demonstrated so far in other animals than great apes. Further comparative studies need to be 
conducted to get a better view of the animal species capable of probabilities estimation as their 
number may be far larger than thought. This is consequently a paper of high significance for the field. 
I nevertheless regret that the study did not put absolute numbers and relative frequencies in conflict 
(instead of equating one) to further understand the mechanisms followed by the keas: do they 
naturally favour relative or absolute numbers? Here, the results show that they are able to use 
relative frequencies if necessary to solve a task but not that this is their favoured method as seems to 
be the case in Humans. Additionally, the number of tokens used are quite large thus making harder to 
process absolute numbers at the benefice of relative proportions. Why not using lower numbers? A 
control experiment demonstrating their ability to discriminate and compare such numbers would be 
ideal but at the minimum, information about the current knowledge on keas’ numerical abilities should 
be provided to convince about their faculty. From a practical point of view, the images provided 
suggest that, in case of high numbers, many tokens would be hidden by the others which makes an 
absolute counting harder. I suggest to provide real photographs of the jars if this was not the case. 
Minor suggestions: 
1. It should be stated in the results section of experiment 1 that the initial performances are recorded 
over the 20 first tests. 
2. I do not understand why 63 and 58 tokens were used in the condition 3 of the first experiment 
instead of the exact same number. Could you explain? In addition, given the number chosen (3 
rewarding token for 57 unrewarded), the rewarded tokens were probably hardly perceivable, thus 
making the conclusion of the whole experiment less convincing. 
Table 1: Could you add below the pictures the proportions and exact numbers of each category of 
tokens ?