The main reason why kea tampered with baited trap-boxes was to access the egg bait, both inside and outside our study area. Our video footage demonstrated that the repeated stick insertion and probing by the kea at LC02 was purposeful to interact with out-of-reach trap-box contents. Also, the kea at TM20 was intent on looking into the trap-box. Kea are known to persist at exploratory play with an object21, but the obvious play at LC02 did not involve stick insertion, or probing, into the side entrances. Thus the kea’s stick probing at LC02 was purposeful and clearly tool use8,40. The tool use has important implications for the unobserved insertion of objects (i.e. sticks and wire) into trap-boxes both inside and outside the Murchison Mountains study area. Kea also probed into unbaited run-through traps-boxes in 2014 (Fig. 1A). This is curious given that they seemed to have previously ignored trap-boxes without egg bait several years earlier. Thus kea might have probed into run-through trap-boxes that they probably knew were unbaited because the effect of setting off snap-traps was intrinsically rewarding. They may have done so as the snap-traps were relatively easy to reach with sticks through entrances on the ends of these trap-boxes. Further work is needed to determine if kea used tools to simply deactivate snap-traps.

Kea stick tool use described here appears to have been innovated in response to the introduction of trap-boxes. First, tool use by free-living kea was previously unknown in spite of considerable research on their behaviour in the wild21. This is supported by the lack of stick insertion into trap-boxes over many years on Resolution Island and at other sites in and close to Fiordland. Second, the first sticks inserted into trap-boxes were only noticed in the study area well after the initial introduction of (baited) trap-boxes in 2002. If the kea were already proficient at using stick tools, we would have expected to find sticks commonly inserted into trap-boxes soon after their initial installation in 2002 given the kea’s extractive foraging skills and curiosity about human-associated objects. However, as Fig. 3 shows, sticks were recorded inserted into relatively few trap-boxes in the south east region where traps were restricted to from 2002–2007. The original trap-boxes in this region were not modified with side entrances until after 2012 (Supplementary Table S1). Last, the first items noticed inserted into trap-boxes were not sticks but those that were ineffective at setting off snap-traps or aiding bait extraction. Given anecdotal reports of sick insertion into trap-boxes 120 km distant from the study area, tool use by kea to access trap-box contents may have developed independently more than once.

Large variation across years existed in the number of different trap-boxes in the study area that were recorded with sticks inserted. For example, the low number in 2011 contrasts with the much higher numbers in November 2010 and January-June 2012. Some of the variation was due to missing data when staff inspecting trap-boxes did not record stick insertion. Variation may also be associated with the series of modifications to the design of trap-boxes to try and prevent kea tampering (Supplementary Table S1). The initial outbreak of stick insertion in 2009/2010 was quickly followed by the fitting of wooden baffles to the side entrances of trap-boxes outside the south-east region. The baffles likely reduced stick insertion in 2011 before it seems to have increased in January-June 2012. Modifications to trap-boxes such as fitting side entrances also reduced the frequency of kea stick insertion, especially just after their introduction (T.H., personal observation). The widespread tampering with trap-boxes at several Fiordland locations that did not involve sticks (e.g. tipping over trap-boxes, opening trap-box lids) was carried out by many kea. Similarly, the widespread stick insertion across our study area suggests that more than one kea were inserting sticks into trap-boxes. Data on kea movements show that individual kea could potentially forage throughout much of the Murchison Mountains. Daily activity patterns of two banded kea at Mt Cook National Park included flights of 10–20 km29. Studies41,42 in the central and upper South Island also found that banded kea were usually resighted 10–20 km from their banding location. Juveniles, though, probably disperse from their natal range when independent42. Nevertheless, the extent, geographic spread and persistence of stick insertion into trap-boxes over many years makes it more likely that multiple kea were habitually carrying out this behaviour.

At first glance, our findings might be viewed as support for the predominant role of ecological conditions in initiating tool use. For example, there was no ecological opportunity for kea to use probing tools before the trap-boxes were installed. The trap-boxes provided that opportunity and tool use was invented. Similarly, food supplies could have been unusually scarce around the time kea began inserting sticks into trap-boxes, thus necessity together with opportunity might have initiated tool use. Few data exist on how kea food supplies varied in the study area. However, many plants in New Zealand forests, including the extensive beech forests (family Nothofagaceae) in the Murchison Mountains32 (kea are known to occasionally eat beech seed29), are masting species and do not produce an abundance of sound seed every year43. Therefore, beech mast years in our study area may provide some indication of weather-related variation in a range of food resources for the birds. There were four mast years between 2000 and 2015: in 2006/2007, 2009/2010, 2011/2012 and 2014/2015 (Sanjay Thakur, personal communication). This means there were at least four non-mast years after trap-boxes were first installed in 2002 before kea began to probe into trap-boxes with sticks. Thus the initial appearance of kea tool use around 2009/2010 does not appear to be closely associated with extremely low food resources in their habitat (necessity hypothesis).

Cognitive constraints may better explain why kea did not appear to have invented tool use when foraging at our study site before the introduction of trap-boxes. Our discussion below focuses on cognitive constraints and the invention of tool use at the individual level and not on evolutionary explanations. First, Kea practice extractive foraging and are highly motivated to interact with potential probe sites. In captivity they are much more interested in a cube with a slot that can be probed with the bill or looked into than one without a slot21. In their sub-alpine forest habitat, there should be potential probe sites with cached food (e.g. invertebrates, lizards) where it would be more efficient to employ tool use. Although it has a bill specifically evolved for fine tool manipulation44,45, the New Caledonian crow uses tools extensively to extract prey cached in forest habitat from near sea level to over 1,600 m above sea level46,47. Thus it is highly likely that opportunities exist for kea to invent tool use to extract prey from natural probe sites. Furthermore, the probing by New Caledonian crows is often more investigative and does not appear to require precision manipulation of the end of the tool (G.R.H., personal observation). We have shown here that kea have the manipulation skills to carry out this kind of tool use. Bills of female kea are 13% shorter than those of males48, thus tool use by females in particular may be advantageous for obtaining out-of-reach prey. Second, the design of the initial trap-boxes facilitated the opportunity for innovating tool use compared to the characteristics of natural holes and crevices. The trap-boxes made it relatively easy for a bird with high object manipulation skills to get a successful outcome by inserting a stick (i.e. access to food). Trap-boxes with end entrances in wide mesh, in particular, enabled kea to easily insert sticks of a range of sizes while having a clear view of the ‘cached’ contents (i.e. food and snap-trap) and stick movement. In contrast, potential prey cached in natural probe sites are unlikely to be in full view of kea. Side entrances where kea could not see trap-box contents while probing were probably more realistic of natural probe sites. However, probing into side entrances likely occurred after previous experience of probing into the ends of trap-boxes. The trap-boxes were also at high density, visually obvious and regularly baited over many years, which provided the opportunity for kea to repeat and maintain the tool use. Third, the lack of tool use by kea to probe into trap-boxes outside the study area is inconsistent with the ecological opportunity hypothesis. Kea probing into trap-boxes with sticks across several years in the Murchison Mountains suggests that tool use was often advantageous for obtaining egg bait. Their behaviour at other locations, such as biting through mesh to extract egg bait and tipping over trap-boxes, demonstrates that the kea were well motivated to interact with trap-box contents. Thus those trap-boxes clearly provided an opportunity for kea tool use but evidence of the behaviour only consists of anecdotal reports.

To recall, natural probe sites likely exist for kea where tool use to extract prey would be more efficient than bill use, even if they were uncommon. The trap-boxes at our study site greatly facilitated tool use, but even so the behaviour did not emerge until many years after their installation. This reinforces our contention that innovating tool use at natural probe sites is cognitively demanding. Confirmation that the trap-boxes facilitated the innovation of kea tool use as opposed to providing a unique opportunity for the behaviour would be if kea transferred the tool use to natural probe sites when extractive foraging. Therefore, follow up work on natural foraging by kea in the Murchison Mountains could provide important insight into why tool use is rare in animals. That tool use emerged at our study site but has been rarely reported at other trapping locations suggested that some kea have a level of domain-general intelligence for inventing tool use while others do not, which also implicates cognitive constraints. Under the profitability hypothesis4, when tool use is more efficient, on average, than bill use it will evolve to become population-level behaviour. For example, a genetic basis for tool use is evident in several tool-using bird species, including the New Caledonian crow4,44, but there are currently few data to suggest that tool use is under selection in nonhuman mammals49. Our findings raise the distinct possibility that tool use could persist as occasional behaviour in certain species without coming under selection. Under this scenario, kea with the domain-general intelligence to invent or socially acquire the behaviour would use tools at those less common probe sites where bill use was ineffective. That only some kea appear to have this level of domain-general intelligence even when the opportunity for tool use is greatly facilitated, combined with the increased cognitive challenges of inventing tool use at natural probe sites, would place substantial cognitive constraints on tool use emerging in the course of natural foraging.

Field experiments with honey extraction also implicate cognitive constraints as the main reason for the lack of stick tool use by some primate populations who only use other types of tools. The Sonso population of chimpanzees in Uganda that use leaf tools but not stick tools failed to use sticks to extract honey in prepared holes50,51. In a similar experiment, the Pedra Furada group of bearded capuchin monkeys (Sapajus libidinosus) in Brazil who use stone tools but not stick tools also failed to use sticks to extract honey from holes52. When the degree of facilitation was increased by leaving sticks in the holes for individuals to pull out and use as tools, individuals from both populations still failed to use the sticks as tools51,52. Gruber and colleagues3,50,51 have suggested that conservatism or cultural bias for existing, non-stick-tool behaviour and/or ‘functional fixedness’ (failure to use familiar objects in novel ways) may have prevented stick tool innovation in Sonso chimpanzees. Similar work with captive, non-tool-using macques (Macaca fascicularis fascicularis) failed to elicit stone-hammering tool use to access nut contents in 31 motivated individuals53. This was in spite of increasing levels of facilitation that culminated in a full tool use demonstration by their human keeper. Further research is needed to identify the cognitive constraints that can prevent experienced tool-users inventing novel tool use. Nevertheless, the above examples lend support to the view6 that cognitive constraints rather than associated ecological conditions are the crucial factor limiting the innovation of flexible tool use in nonhuman animals. Our study was similar to the capuchin and chimpanzee field experiments as kea could also decide on their own whether or not to participate, but it was an unplanned one. Unlike the primate experiments above, the degree of facilitation for tool use was not increased for kea. On the contrary, it was mostly reduced in response to kea success at inserting sticks into trap-boxes and setting off snap-traps.

In summary, we provide the first evidence that a non-tool-using bird species with a high level of physical cognition innovated habitual tool use in the wild in response to facilitation. This finding is consistent with a prediction of the cognitive constraints hypothesis explaining why flexible tool use is rare. That is, birds with a high level of intelligence and physical cognition, like the kea, should more readily innovate tool use when it is facilitated. We suggest an experimental paradigm to test this prediction, where non-tool-using species that differ in cognitive ability would be required to innovate tool use that is facilitated to varying degrees. The expectation would be that the level of physical cognition and object manipulation skills will be negatively correlated with the degree of facilitation needed for tool use to emerge. Furthermore, our findings indicate that even with a high degree of facilitation the invention of tool use in the wild for species like the kea with considerable domain-general intelligence is cognitively demanding.