Flexibility in Problem Solving and Tool Use of Kea and
New Caledonian Crows in a Multi Access Box Paradigm
Alice M. I. Auersperg1,2*, Auguste M. P. von Bayern3, Gyula K. Gajdon1,2, Ludwig Huber1., Alex
1Department of Cognitive Biology, University of Vienna, Vienna, Austria, 2Konrad Lorenz Institute for Ethology, Vienna, Austria, 3Department of Zoology, University of
Oxford, Oxford, United Kingdom
Parrots and corvids show outstanding innovative and flexible behaviour. In particular, kea and New Caledonian crows are often
singled out as being exceptionally sophisticated in physical cognition, so that comparing them in this respect is particularly
interesting. However, comparing cognitive mechanisms among species requires consideration of non-cognitive behavioural
propensities and morphological characteristics evolved from different ancestry and adapted to fit different ecological niches. We
used a novel experimental approach based on a Multi-Access-Box (MAB). Food could be extracted by four different techniques,
twoof theminvolvingtools.Initially all four optionswere available to the subjects. Once they reached criterion for mastering one
option, this task was blocked, until the subjects became proficient in another solution. The exploratory behaviour differed
considerably. Only one (of six) kea and one (of five) NCC mastered all four options, including a first report of innovative stick tool
use in kea. The crows were more efficient in using the stick tool, the kea the ball tool. The kea were haptically more explorative
than the NCC, discovered two or three solutions within the first ten trials (against a mean of 0.75 discoveries by the crows) and
switched more quickly to new solutions when the previous one was blocked. Differences in exploration technique, neophobia
and object manipulation are likely to explain differential performance across the set of tasks. Our study further underlines the
need to use a diversity of tasks when comparing cognitive traits between members of different species. Extension of a similar
method to other taxa could help developing a comparative cognition research program.
Citation: Auersperg AMI, von Bayern AMP, Gajdon GK, Huber L, Kacelnik A (2011) Flexibility in Problem Solving and Tool Use of Kea and New Caledonian Crows
in a Multi Access Box Paradigm. PLoS ONE 6(6): e20231. doi:10.1371/journal.pone.0020231
Editor: Warren H. Meck, Duke University, United States of America
Received February 10, 2011; Accepted April 13, 2011; Published June 8, 2011
Copyright: ? 2011 Auersperg et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Involved in funding the kea studies were the FWF; Fond zur Fo ¨rderung der Wissenschaftlichen Forschung in O¨sterreich (Project 19087 to Ludwig
Huber) as well as the EU (ERC grant SOMACCA to T. Fitch). The crow studies presented were supported by the ESF; European Science Foundation. The funders had
no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: email@example.com
. These authors contributed equally to this work.
Cognitive mechanisms evolve to a large extent in response to
selective pressures peculiar to each species’ ecology, physiology
and morphology . Because of this, cognitive processes may
differ qualitatively and quantitatively across species.
One important tool of comparative cognition is to analyze the
performance of different species facing the same task. However, if
two species are to be compared on the basis of their final
performance on a single task then task-specific factors may lead to
differences that are not indicative of general problem-solving
ability but are instead expressing different motivational predispo-
sitions or other non-cognitive specialisations.
A well-argued response to this problem is using a battery of
different tasks rather than a single one [2–6]. It is also essential to
use a sufficiently diverse set of tasks. For instance, comparing tool-
using and non-tool using species only in tool-using tasks and
reaching conclusions in terms of problem-solving ability would
make little sense, as would using only tasks that favour proactive
rather than passive solutions.
We then start from the premise that focusing on species-specific
mechanisms and on the strategies that underlie problem-solving
performance in a diversity of contexts is more informative than
attempts to rank individuals or species on problem-solving success
without due reflection on the cognitive demands of the tasks
employed. This logic could be followed by offering several tasks
simultaneously and by successively removing the options that have
already been mastered. A survey of discrepancies between species
of which tasks are approached first, how many solutions are
discovered, how quickly the species re-learn, overcome habits and
adapt to changes by switching between different solutions, may
expose factors influencing species-specific traits such as behav-
ioural flexibility, neophilia, exploration strategy, attention, moti-
vation, affordance learning, anatomical constraints and other traits
of interest. Apparatuses offering different physical problems at the
same time have been used in various experiments targeting intra-
specific social learning (rather than technical competence) eg. [7–
10], but to our knowledge, have not been used for inter-specific
comparisons of the mechanisms underlying problem solving.
We applied this approach to two highly competent extractive
foragers: a parrot (the kea, Nestor notabilis) and a corvid (the New
Caledonian crow, Corvus moneduloides). These birds are both
representatives of two avian families that stand out for having larger
encephalization quotients [11–13] and greater innovation scores
[14–16] than other avian taxa, and that seem to parallel the great
apes in performance in some physical tasks [17–26]. Kea are
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neophilic mountain parrots of New Zealand with well-known
manipulative skills that are not known to use tools in the wild, while
New Caledonian crows are known for their tool manufacture and
use inthe wild [27–29]. Both species are renowned for their problem
solving skills in various physical cognition tasks [8,17,18,20,23–
26;30–32]. Both species are generalists that live in social groups and
within complex environments with fluctuating resources and pursue
a foodextracting foragingstyle,keadiggingforrootsinthe ground of
mountain plains and crows fishing for larvae in decaying tree logs
[27,29,33].Ourcaptive kea hadalsoshowncompetenceinthe use of
compact objects, such as wooden blocks, as tools to access rewards
[18,30]. This gives us an opportunity to disentangle various possible
cognitive and non-cognitive factors influencing differential perfor-
mance in problem solving in a natural tool user as well as a naturally
non-tool using species.
One of the issues that motivate our study concerns the evolution
of tool use and its cognitive underpinnings, in particular whether
there is something special about the cognition associated with tool
use and other forms of complex object manipulation. For example,
it is possible that adaptive specialization to a given type of tool use
may either enhance or impair performance in other tool-related or
object-manipulation tasks. It has been argued, that while tool-
related behavior may not necessarily be more cognitively
demanding than other forms of problem-solving, it may be more
revealing of the information-processing that it involves, and hence
may be useful to expose what animals understand about the
relationship between objects and the effects objects have on one
another . So far researchers have attempted to experimentally
determine the role of factors such as pre-functional development
[28,34] (associative) experience, affordance learning , self-
control, planning and reasoning about invisible forces . Here
we focus on how the specificity of tasks determines differential
performance in a comparative setting.
We developed a ‘‘Multi Access Box’’ (MAB), as a tool to
compare problem solving in extractive foraging species. The MAB
features a battery of tasks that all lead to the same goal, a food
reward presented in the centre of a transparent box. Initially the
subject is allowed to choose any of the four options, but once a
subject has developed a consistently successful performance with
each technique to access the food, this solution is blocked and we
record its performance in establishing competence in an
alternative strategy. Six kea and five crows were exposed to this
setup, where two of the tasks required the use of a mediating object
as a tool (either a stick or a ball). The other two solutions did not
require tool use and involved either pulling a string tied around the
reward or pulling from a hook handle to open a window.
There were differences in the way kea and crows behaved
towards the apparatus (see apparatus in Figure 1), and this
translated in differences in the number of solutions discovered
(namely used successfully at least once) within the first session.
Averaging across individuals, NCCs found 0.75 (Range 0–1) and
kea 2.33 (Range 2–3) solutions within the first session (see Figure 2).
Also within the first session, all kea touched (i.e. made physical
contact with) all four opening devices and both tool types, while
the NCCs rarely touched the apparatus (except the string). The
crow Annie-Claude was excluded from testing after failing to
retrieve the food reward within the time given during the
Figure 1. The Multi-Access-Box (MAB). Notice the four exchangeable transparent walls with openings corresponding to the 4 possible solutions
(string, window, ball and stick). Dimensions in cm.
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The kea directly manipulated the opening devices more often
than the crows did in the course of this study: When repeated
touchings are considered, the mean percentages of trials in which
five or more manipulations of an entrance occurred were 18% +/
21.92 SE, N=6 in the kea than in the crows (Mann-Whitney U
test; Z=2.34; p=0.01). Entrance manipulations by the crows
consisted almost exclusively of brief pecking actions at the
openings with the beak tip or with a tool, while the kea’s
manipulations included violent pulling and tearing as well as
rocking, probing, scratching, and levering of the physical parts of
Figure 2. First discoveries of the various solutions. A) Mean number of first discoveries throughout sessions (the last discovery was the
window solution by the NCC Uek in session 13). Kea are represented by the green line; NCC by the red. B) Mean number of trials until a solution was
discovered. Green = kea; red = NCC. T-bars represent SE.
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Sequence of solutions discovered and established
After an initial period distributing their attention broadly
around the apparatus, all subjects, kea and crows, focussed on the
string option. After the string solution was blocked, the subsequent
order of solutions established differed between the two species and,
in kea, between individuals (see Table 1).
Three NCCs next reached criterion using the stick tools
(between session 4 and 12) and, when this option was blocked,
two of these reached criterion with the ball tool (in sessions 8 & 13;
see Table 1). Finally, one of these two crows (Ue ´k) solved the
window hook task. In many cases the crows touched the window
(which could not be opened that way) with a stick, rather than
trying to pull the hook. On average 31.77% +/28.00 SE, N=3 of
the ineffective tool actions (ITA= inserting or bringing the tool
into contact with an inappropriate opening; note that in an ITA a
tool has to touch an opening and not just any part of the
apparatus) in the trials in which the crows finally used a tool to
retrieve the reward were directed against the window option. In
fact, after learning to open the window by pulling from the hook,
Ue ´k always reached for a stick and used it to poke the reward out,
instead of sticking her head through the window and taking the
reward directly, as the kea did (see Movie S2).
The kea met criterion in either the ‘ball’ or the ‘window’ option
after the string solution was blocked (all kea finished these three
solutions). After these options were closed as well, all attempted to
use a stick tool, touching the box several times at the appropriate
side with the stick (i.e. carrying the tool to the correct opening and
touching it with the tool), but failed to insert it. Only one kea,
Kermit, succeeded in developing a successful technique to retrieve
the reward with the stick. Due to the curvature of their beaks, kea
cannot hold a stick in alignment with their heads to the same
degree as New Caledonian crows and as a consequence have less
control over the tip of the tool. Kermit developed a special routine.
He (1) took the tool end laterally into his beak and pushed that end
into or against the tool entrance. (2) He then switched from
grabbing the stick with the beak to grabbing it with the foot,
continuing to press the tool end against the opening, (3)
Meanwhile he shifted the beak to the end of the tool that was
distal to the opening securing the tool’s position with the foot at
the tool entrance. Finally (4) he directed the tool through the
opening with his beak and manoeuvred it until it hit the reward
(see Movies S1, S2). Kermit successfully used the stick tool for the
first time three sessions after all other openings had been closed (in
session eight) and reached criterion in session ten.
Speed in switching from one solution to the next
The kea tended to be faster than the NCCs at meeting criterion
in new solutions once the previous method was blocked. They took
less sessions on average (1.3 +/20.21 SE, N=6) to switch from the
first to the second option than the three crows that did master a
second option (which took 4, 11 and 2 sessions; Mann-Whitney U
test; Z=2.19; p=0.048). The kea took on average 1.67 +/20.21
SE, N=6 sessions to switch from option 2 to option 3 while the
two crows that mastered a third option, took 4 and 5 sessions
respectively (Mann-Whitney U test; Z=2.145; p=0.071). Only
one subject of each species mastered a fourth option. The crow
took 4 sessions to reach criterion from the ball (3rdoption) to the
window (4thoption), while the kea took 5 sessions to reach criterion
from the ball option (3rdoption) to the stick (4thoption; see
Table 1). Establishing the stick solution after the string solution
had been blocked, took the three successful crows 5.8 sessions on
average even though the task taps into their proven skills as natural
tool users (Table 1 & Figure 2).
Tool preference and other analyses related to the tool
The crows only successfully used the thin sticks and the single
successful kea used only the thick sticks. Both species used both
sizes of ball tools.
After the string opening had been blocked, the mean number of
times the kea brought a ball tool into contact (by touch or
insertion) with the apparatus’s openings was significantly higher
than with a stick (Wilcoxon signed-rank; T=2.02; p=0.043).
After the ball option was blocked, balls were removed after three
unsuccessful sessions in which all options except for the stick
option were blocked for 5 of the 6 kea, because they continued to
make attempts to use these now non-functional objects as tools.
Nevertheless, no additional kea managed to reach the food with a
stick. For the crows the reverse situation was true; they tended to
touch openings with the sticks more often than with the balls.
However, this was only significant at the 6% level (Wilcoxon
signed-rank; T=1.87; p=0.06).
Table 1. Order and session in which each individual reached criterion (8 consecutive times correct or 9 out of 10 correct) for each
of the four solutions (string, window, ball and stick).
SpeciesInd1stsolutionSession2ndsolutionSession3rdsolution Session4thsolution Session
Kea BrString3 Window4Ball6--
Fr String2 Window3Ball5--
Ue ´k String2Stick4Ball8Window 12
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The three crows that did reach criterion using tools tried to
insert the stick more often into openings than the six kea (Mann-
Whitney U test; Z=2.59; p=0.01) whilst the kea combined the
ball more often than the crows (Mann-Whitney U test; Z=1.9;
We find similar preferences looking only at trials in which
ineffective tool actions occurred (see Figure 3). The kea brought the
trials in which the performance was immediately successful) than the
NCCs (Mann-Whitney U test; Z=2.32; p=0.024).
In trials including ITAs, the kea also inserted the ball into
openings such as the string or the stick opening rather than the
window, while the NCCs also touched the window opening
frequently with a stick tool (see Figure 3). In incorrect tool use
trials, in which the animals conducted ineffective ITA before
succeeding to insert the tool appropriately, the average number of
ITA per trial was similar in crows and kea (see Table 2).
At least one individual of each species discovered all four
available methods. This proves that in principle, the affordances of
the tasks lay within the cognitive and physical capacity of both
There were, however, interesting differences in performance
with the present set of tasks. The kea were faster in discovering
multiple solutions and showed more individual variation than the
naturally tool-using NCCs. Within the first session, all kea
successfully employed at least two or three solutions while none
of the crows used more than one. The kea also switched to other
solutions quicker once previously mastered solutions were blocked.
Although both species had experience with compact objects
[18,24,30], using the ball was acquired faster by the kea, while in
the stick option the naturally stick-tool using NCCs were faster.
Only one kea succeeded in inserting the stick tool into the correct
opening, although all attempted to do so.
To a large extent, differences in exploration patterns and
affordance learning as well the balance between neophilia/
neophobia seem to be responsible for the differential performance.
The kea showed more haptic exploration while the crows,
probably due to their higher level of neophobia, seemed to
explore more in a visually guided manner. Similar differences exist
between kea and common ravens . The kea’s higher readiness
to manipulate i.e. act on novel objects, may help them to detect
functional affordances. In their naturally low-risk, variable
environment, neophilia may reflect low predation risk [20,37].
The crows, in contrast, approached the apparatus hesitantly, and
touched it less often than the kea. Two crows never explored the
box thoroughly with their beaks. One of them had to be excluded
from the study because it never approached the experimental
setup. Neophobia hampered the crows’ performance in other
respects. For instance, despite their predisposition, experience and
Figure 3. Mean (± SD) number of Ineffective Tool Actions (ITA)/trial throughout all trials in which performance was not
immediately successful (excluding data in which of the two tools was removed); green bars =kea, red bars = NCCs; e.g. Stick-Ball
indicates the mean number of times the stick was brought in contact with the ball entrance per trial (in which performance was not
Table 2. The frequency of Ineffective Tool Actions (ITA) in
trials in which either the ball or the stick was used to retrieve
Kea Fr1.380 100
Br 1.27 17.8582.14
Ke 0.6123.0776.920.45 33.33 66.67
Lu 0.40 100
Ta 0.58 14.2885.71
NCC Ey 1.06 79.4120.59 0.521000
MeanKea 0.9310.9889,01 0.4533.3366.67
This table depicts the mean number of ITA per trial in which the reward was
finally retrieved using the ball tool (column 3) or the Stick tool (column 6) as
well as the % of ITA in which the stick (or the ball respectively) was inserted into
inappropriate openings before succeeding with one of the two tools.
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competence for stick tool use, the crows did not use the available
tools as first option and also took some time to use tools after the
string pulling option had been removed. (The string option was the
first to be blocked for both species. It may have represented one of
the more conspicuous affordances of the apparatus or evoked little
neophobia since both species were familiar with strings; see
methods). It is possible that because the available sticks were
smoother, painted, and considerably thicker than those the crows
naturally choose in the wild  ergonomic difficulties synergised
with neophobia, and may have affected their performance. Once
the crows had established stick tool use, they continued to use the
sticks for exploring the box instead of touching it directly. This is
reflected in the number of ITA with sticks when confronted with
the ball and window option, i.e. after the stick option was blocked.
This may indicate a preference to explore unfamiliar objects with
tools rather than touching them directly, as shown by Wimpenny
et al. (2010) . Such behaviour is, however, disadvantageous in
situations where the functionalities of the objects are not detectable
by simply applying pressure, such as in the case of the window
option, illustrating one cost of the otherwise useful capacity to use
tools for exploratory goals.
Another strong difference was that the kea showed greater
destructiveness than the crows, as a consequence of their forceful
and frequent use of pulling and tearing actions. One kea, Luke,
even broke the PlexiglasH on the top of the box while trying to
force it open, and most kea attempted to turn over the MAB,
which had to be fixed to the aviary floor. Wild kea are well-known
for what human observers usually describe as curiosity, playfulness
and urge to tear apart objects such as cars’ windshield wipers or
picnic baskets .
Wild (as well as naı ¨ve captive) NCCs also use tearing actions
when manufacturing tools from pandanus leaves [29,40]. They
however did not use such behaviours in this setup. We can at this
point speculate that tearing behaviours may mainly be orientated
towards tool making and nest building rather than to exploration of
novel objects or food extraction. The slight curvature at the beak tip
of most corvids has been interpreted as an adaptation to feeding on
carcasses . NCCs might have secondarily lost their beak
curvature making their beaks less suited for violent tearing actions.
Very recently Rutz et al. (2010)  were able to show that a
substantial amount of the crows’ protein and lipid intake came from
wood boring beetle larvae obtained with stick tools, also indicating
seems therefore possible that the range of exploration techniques
during foraging in NCC may be constrained by the adaptive
specialization for tool use and that this will affect the ‘‘zone of latent
solutions’’  within the species’ cognitive repertoire.
Our results also provide the first experimental evidence of stick
tool use in a parrot. Kea are neither natural tool users like New
Caledonian crows, nor do they construct nests with twigs as all
corvids do, thus lacking the predisposition of nest builders for
handling twigs and other elongated stick-like objects . Instead,
kea use or dig burrows for laying their eggs . Also importantly,
ergonomically, the use of sticks is clearly difficult for kea. The
curvature of their beaks and pronounced size difference between
upper and lower beak, precludes a good grip and control of long,
straight tools. NCCs, maybe as an adaptation to tool use ,
have short straight beaks with the mandible almost as long as the
maxilla, allowing them to effectively hold sticks directly forwards,
functionally elongating their beaks and increasing their reach.
To overcome these difficulties, the single successful individual
kea developed a complicated stepwise technique, involving
carefully concerted foot and bill actions (see Movie S1). This
permitted the subject to insert and direct a stick tool despite the
species’ morphological constraints. Kermit’s performance indi-
cates a high degree of deliberate control over his movements,
suggestive of anticipation of their effect and perhaps a represen-
tation of the goal action, i.e. of inserting the stick into the opening.
Proof of goal directedness or goal representation, however,
requires specific tests that were not implemented here .
Our study illustrates the difficulties of comparative cognition
research and points to some partial solutions. Clearly, no single-
task exploration can be used to assess problem-solving ability or
make claims for advanced general intelligence or innovativeness.
This caveat applies to within as well as between species
comparisons. Problem solving is intrinsically multi-dimensional
and it is to be expected that individuals or species will outperform
each other in different dimensions. Batteries of tasks designed with
this in mind may however be highly informative about the
different predispositions and cognitive competences across indi-
viduals and species.
Six male kea (Frowin, Kermit, Pick, Tammy, Bruce and Luke),
as well as five New Caledonian crows (Boycott, Annie-Claude,
Tino, Ebony and Ue ´k), two of which were male (Boycott and
Tino) participated in this study. Three kea (Kermit, Pick and
Tammy) as well as one crow (Uek) were hand-raised. Bruce, Luke
and Frowin were parent-raised in captivity; Annie-Claude, Tino,
Ebony and Boycott were wild caught but had laboratory
experience. Bruce and Luke were seven, Frowin, Pick and Kermit
five, Tammy three and Uek, five years old. All subjects had
experience in experimental problem-solving setups substantially
different from that used here [8,17,18,24,30,47,48]. The NCC had
participated in a test where they dropped stones into vertical tubes
. Kea also inserted objects into vertical tubes (unpublished
data) and participated in follow up tool-use experiments with loose
rewarded tubes or tubes in a slanted position and compact tools
[18,30]. Both the NCC and the kea had experiences with strings:
the kea had participated in a vertical string-pulling task [32, and
unpublished data] and the crows were given experience, pulling
30 cm long strings (of different colour and diameter as during
testing) up a perch prior to a yet unpublished study. The crows
had experience with stick tools within experimental and natural
The kea were housed in a large outdoor aviary (15 m 610 m
64 m)ina grouptotalling20kea,whilethecrowswerekept inpairs
in outdoor aviaries of various shapes, with an average volume of
approximately 50 m3and access to heated indoor divisions (ca.
8 m3), with food and drinking water ad libitum. The experiments
were strictly non-invasive and based purely on behavioral tests. All
subjects were housed in accordance with Austrian and German law.
We designed and used a Multi-Access-Box (MAB) consisting of
a cubic box (23 cm by side; for details and further dimensions see
Figure 1, see additionally Figures S1, S2, S3, S4, S5; Movie S2)
with four exchangeable transparent walls (the dimensions were
adequate for both species). Each wall contained an opening that
could be used to access a food reward presented in the centre of
the box, either directly or by means of a tool. The food reward
(half a peanut in its shell for the kea; a mealworm inside half a
peanut shell for the NCC) was positioned on a vertical pole in the
centre of the box, which was attached to a slanted platform. Once
the food fell off the pole it rolled down the platform and out of the
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There were four possible solutions (‘string’, ‘window’, ‘ball’ and
‘stick’) through which the out-of-reach food could be obtained, two
of which (‘ball’ and ‘stick’) involved the use of tools. (i) The ’string’
solution required the subjects to pull a string (20 cm) hanging out
of the opening in the respective wall, the other end of which was
tied to the reward. (ii) In the ‘window’ option, a hinged window in
the sidewall of the box could be opened by grasping a hook-like
handle with the beak, pulling he window open and thereafter
reaching into the box to retrieve the food from the pole (or pushing
it off the pole with a stick). (iii) To exploit the ‘ball’ option, a
compact object (a marble) had to be inserted into the respective
opening, which connected to a transparent tube bending towards
the central pole. When inserted into the tube the ball rolled down
the chute and knocked the reward off the pole. (iv) Finally, the
opening corresponding to the ‘stick’ solution was connected to a
(8 mm) short straight horizontal tube at the same height as the
food, but with a ten centimetre gap to the pole (Figure 1). Here,
the food could be obtained by inserting a stick tool in the correct,
hence unobstructed, opening, manoeuvring it towards the pole
and hitting the peanut (kea) or stuffed peanut shell (NCCs).
All openings except for the window had the same round shape
and diameter, hence were superficially perceptually similar, but
could be distinguished by the visible internal structure (see
Figure 1). Four sticks (15 cm long) and four balls, all painted with
yellow childproof acrylic varnish, were provided in two different
sizes. Two of the four supplied sticks were broad and two thin
(0.5 cm and 1,5 cm in diameter respectively), whilst the balls were
two large and two small marbles. The potential tools were placed
in the four corners of the MAB so that each corner had one stick
and one marble (the possible combinations of tool diameters were
randomly assigned). The MAB was turned around before each
trial and the walls were switched; so that the openings were at
randomly changing positions.
Prior to the start of the actual tests, the birds received at least
four familiarization trials, in which each of the walls was missing
once so the birds could just reach into the box and take the food
reward. The crows, which are more neophobic than the kea,
received as many familiarization trials as necessary to recover the
reward in less than three minutes.
During testing, subjects were visually isolated from their group/
mates and received a maximum of ten trials per session. A trial
continued until the reward was recovered or until ten minutes had
passed. If an animal did not obtain the reward within ten minutes
testing continued the following day. A trial was scored as correct if
the bird obtained the reward by applying one of the four solutions
described above without prior unsuccessful attempts to solve the
problem in a different way, e.g. previously manipulating other
openings or combining the tools with wrong openings. The first
successful retrieval of the food from a new opening (the bird may
have manipulated other entrances in the same trial) was scored as
Initially all openings were available. Once a subject reached
criterion for one solution (obtaining the reward by always using the
same solution for two consecutive sessions, but also after nine
correct trials within one session (of ten trials) or eight consecutive
correct trials using the same solution within one session), the
respective opening was blocked (the window was cemented into its
frame, the string was removed and the tool entrances were blocked
with a wooden stopper), so as to force the subject to shift to other
solutions. Testing continued until the animals failed to recover the
reward within ten minutes in three consecutive trials or until all
openings were closed. In order to determine whether each species
had the capacity for each tool option, if just one of the two tool
openings remained open, and the birds failed to solve it three
consecutive times, we gave them a ‘second chance’ (another three
trials) and removed the tools that belonged to the tool task they
had previously solved, so as to remove possible distracting factors.
We reasoned that the birds may fail to solve a tool task simply
because they might not be able to inhibit using a tool which has
been strongly associated to food before.
All data was videotaped. We used SPSS for statistical analysis.
by the male kea Kermit to insert the rod shaped tool into the
appropriate opening as described in the results section.
This video shows the complex motor technique used
and New Caledonian crows, employing all four different solutions
of the Multi Access Box apparatus: string, ball, stick and window.
The video illustrates the techniques used to employ the fourth and
final solution of the two animals, the kea Kermit and the crow
Uek, that mastered all tasks: the crow Uek uses a tool to poke the
reward off its platform after opening the window solution and the
kea Kermit uses a complex multi step technique to insert the stick
tool into the appropriate opening (as described in detail in the
This ‘Kea-crow suite movie’ shows both species, kea
This image depicts a kea inserting a ball tool into the
a ball tool into the appropriate opening.
This image depicts a New Caledonian crow inserting
reward from the window opening using a stick tool.
This image depicts the crow Uek retrieving the
This image depicts a kea opening the window
the appropriate opening.
This image depicts a kea inserting the stick tool into
We would like to thank Lukas Auersperg for technical support in
constructing and drawing the apparatus and Sabine Tebbich for useful
comments on a previous version of the manuscript.
Conceived and designed the experiments: AA AB GG. Performed the
experiments: AA. Analyzed the data: AA. Contributed reagents/materials/
analysis tools: AA AB GG. Wrote the paper: AA AB GG LH AK.
1. Shettleworth SJ (2009) Cognition, Evolution, and Behavior. Second edition.
New York: Oxford University Press.
2. Balda RP, Kamil AC (1989) A comparative study of cache recovery by three
corvid species. Animal Behaviour 38: 486–495.
Problem Solving in Kea and New Caledonian Crows
PLoS ONE | www.plosone.org7 June 2011 | Volume 6 | Issue 6 | e20231
3. Bitterman ME (1965) The evolution of intelligence. Scientific American 212: Download full-text
4. Herrmann E, Herna ´ndez-Llored MV, Call J, Hare B, Tomasello M (2010) The
Structure of Individual Differences in the Cognitive Abilities of Children and
Chimpanzees. Psychological Science 21: 102–110.
5. Kamil AC (1988) A synthetic approach to the study of animal intelligence. In:
Leger DW, ed. Comparative Perspectives in Modern Psychology: Nebraska
Symposium of Motivation Vol. 35. Lincoln. NE. University of Nebraska Press.
6. Lefebvre L, Giraldeau L (1996) Is social learning an adaptive specialization? In:
Heyes CM, Galef BG, eds. Social Learning in Animals: the roots of culture. St
Diego: Academic Press. pp 107–128.
7. Bugnyar T, Huber L (1997) Push or pull: an experimental study on imitation in
common marmosets (Callithrix jacchus). Animal Behaviour 54: 817–831.
8. Huber L, Rechberger S, Taborsky M (2001) Social learning affects object
exploration and manipulation in keas, Nestor notabilis. Animal Behaviour 62:
9. Dawson BV, Foss, BM (1965) Observational learning in budgerigars. Animal
Behaviour 13: 470–474.
10. Whiten A, Custance DM, Gomez J-C, Texidor P, Bard KA (1996) Imitative
learning of artificial fruit processing in children (Homo sapiens) and chimpanzees
(Pan troglodytes). Journal of Comparative Psychology 110: 3–14.
11. Emery NJ, Clayton NS (2004) The mentality of crows: convergent evolution of
intelligence in corvids and apes. Science 306: 1903–1907.
12. Emery NJ (2006) Cognitive ornithology: the evolution of avian intelligence.
Philosophical Transactions of the Royal Society B 361: 23–43.
13. Striedter GF (2005) Principles of Brain Evolution. SunderlandMA: Sinauer
14. Lefebvre L, Whittle P, Lascaris E, Finkelstein A (1997) Feeding innovations and
forebrain size in birds. Animal Behaviour 53(3): 549–560.
15. Tebbich S, Sterelny K, Teschke I (2010) The tale of the finch: adaptive radiation
and behavioural flexibility. Philosophical Transactions of the Royal Society B
16. Overington SE, Boogert NJ, Morand-Ferron J, Lefebvre L (2009) Technical
innovations drive the relationship between innovativeness and residual brain size
in birds. Animal Behaviour 78: 1001–1010.
17. Auersperg AMI, Gajdon GK, Huber L (2009) Kea consider spatial relationships
between objects in the support problem. Biology Letters 5: 455–458.
18. Auersperg AMI, Gajdon GK, Huber L (2010) Kea (Nestor notabilis) produce
dynamic relationships between objects in a second order tool use task. Animal
Behaviour 80(5): 783–789.
19. Bird CD, Emery NJ (2009) Insightful problem solving and creative tool
modification by captive rooks. Proc Natl Acad Sci USA 106: 10370–1037.
20. Huber L, Gajdon GK (2006) Technical intelligence in animals: the kea model.
Animal Cognition 9: 295–305.
21. Pepperberg I (1999) The Alex Studies. Harvard University Press.
22. Tebbich S, Seed AM, Emery N, Clayton NS (2007) Non-tool-using rooks, Corvus
frugilegus, solve the trap-tube problem. Animal Cognition 10(2): 225–231.
23. Taylor AH, Hunt GR, Holzhaider JC, Gray RD (2007) Spontaneous metatool
use by New Caledonian crows. Current Biology 17: 1504–1507.
24. von Bayern AMP, Heathcote RJP, Rutz C, Kacelnik A (2009) The Role of
Experience in Problem Solving and Innovative Tool Use in Crows. Current
Biology 19(22): 1965–1968.
25. Weir AAS, Chappell J, Kacelnik A (2002) Shaping of hooks in New Caledonian
crows. Science 297: 981.
26. Wimpenny JH, Weir AAS, Clayton L, Rutz C, Kacelnik A (2009) Cognitive
Processes Associated with Sequential Tool Use in New Caledonian Crows. PLoS
ONE 4(8): e6471.
27. Diamond J, Bond AB (1999) Kea, Bird of Paradox: The Evolution and Behavior
of a New Zealand Parrot. BerkeleyCA: University of California Press.
28. Kenward B, Rutz C, Weir AAS, Kacelnik A (2006) Development of tool use in
New Caledonian crows: inherited action patterns and social influence. Animal
Behaviour 72: 1329–1343.
29. Hunt GR (1996) Manufacture and use of hook-tools by New Caledonian crows.
Nature 379: 249–251.
30. Gajdon GK, Aman L, Huber L. Keas rely on social information in a tool use
task but abandon it in favour of overt exploration. Interaction Studies, In press.
31. Taylor AH, Hunt GR, Medina FS, Gray RD (2009) Do New Caledonian crows
solve physical problems through causal reasoning? Proceedings of the Royal
Society, London B 276: 247–254.
32. Werdenich D, Huber L (2006) A case of quick problem solving in birds: string-
pulling in keas (Nestor notabilis). Animal Behaviour 71: 855–863.
33. Bluff LA, Weir AAS, Rutz C, Wimpenny JH, Kacelnik A (2007) Tool-related
cognition in New Caledonian crows. Comparative Cognition & Behavior
Reviews 2: 1–25.
34. Tebbich S, Taborsky M, Fessl B, Blomqvist D (2001) Do woodpecker finches
acquire tool-use by social learning? Proceedings of the Royal Society B 268:
35. Emery NJ, Clayton NS (2009) Tool use and physical cognition in birds and
mammals. Current Opinion in Neurobiology 19: 27–33.
36. Schloegl C, Dierks A, Gajdon GK, Huber L, Kotrschal K, et al. (2009) What
You See Is What You Get? Exclusion Performances in Ravens and Keas. PLoS
ONE 4: e6368.
37. Greenberg R, Mettke-Hofmann C (2001) Ecological aspects of neophobia and
neophilia in birds. Current Ornithology 16: 119–178.
38. Bluff L, Kacelnik A, Rutz C (2010) Vocal culture in New Caledonian crows.
Biological Journal of the Linnean Society 101: 767–776.
39. Wimpenny J (2010) New Caledonian crows use tools for non-foraging activities.
Animal Cognition;DOI: 10.1007/s10071-010-0366-1.
40. Kenward B, Weir AAS, Rutz C, Kacelnik A (2005) Tool manufacture by naive
juvenile crows. Nature 433: 121.
41. Kulemeyer C, Asbahr K, Gunz P, Frahnert S, Bairlein F (2009) Functional
morphology and integration of corvid skulls – a 3D geometric morphometric
approach. Frontiers of Zoology 6: 2.
42. Rutz C, Bluff LA, Reed N, Troscianko J, Newton J, et al. (2010) The Ecological
Significance of Tool Use in New Caledonian Crows. Science 329(5998):
43. Tennie C, Call J, Tomasello M (2009) Ratcheting up the ratchet: on the
evolution of cumulative culture. Philosophical Transactions of the Royal
Society B 364: 2405–2415.
44. Hansell M, Ruxton G (2008) Setting tool use within the context of animal
construction behaviour. Trends in Ecology and Evolution 23: 73–78.
45. Jackson JR (1963) The nesting of keas. Notornis 10: 319–326.
46. Heyes C, Dickinson A (1990) The intentionality of animal action. Mind and
Language 5: 87–104.
47. Myiata H, Gajdon GK, Huber L, Fujita K (2010) How do kea solve artificial
fruit problems with multiple locks. Animal Cognition 14(1): 45–58.
48. Liedke J, Werdenich D, Gajdon GK, Huber L, Wanker R (2010) Big brains are
not enough: performance of three parrot species in the trap tube paradigm.
Animal Cognition 14(1): 143–149.
Problem Solving in Kea and New Caledonian Crows
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