Tool-related behavior, especially innovative tool manufacture, is intimately associated with human evolution, and may have co-evolved with specific neurological capacities, particularly planning and complex task coordination1. Innovative tool manufacture emerges only between 5–9 years of age in human ontogeny2,3,4, probably because inventing new tools requires cognitive operations that include executive functions5 that develop only late6 and after extensive individual and social learning. Outside humans, innovative tool manufacture is only known in a small set of taxa, notably in other primates, corvids and parrots (e.g.7,8,9,10,11,12,13).

Here we focus on a particularly rare form of tool innovation, a type of tool manufacture that anthropologists and primatologists consider profoundly significant for understanding human evolution1. Creating maneuverable tools by combining complementary different parts into firmly connected units referred to as compound tools hereafter (also labeled composite tool manufacture1 or additive tool making14), is a particularly rare form of tool manufacture, hitherto unproven outside the hominid lineage. Even among hominids, innovative compound tool construction has been documented only through a few reports in captive great apes7,8,10,13,14 and some authors consider it to be absent in wild chimpanzees (Pan troglodytes)15,16. The latter do, however, display behavior that involves combining different elements into functional assemblages. At Bossou, Guinea, for example, some individuals combine 3 or more elements: they stabilize a relatively large planar rock using a second stone as a wedge, and then use a third stone as a hammer to pound nuts17. The components have specific roles (anvil, wedge, hammer) and are chosen and placed appropriately to make the assemblage functional, but this assemblage forms a tool and substrate composite rather than a compound tool, which is an integrated mobile object14. In another example, at the Sonso community in Budongo Forest, Uganda, wild chimpanzees hold together multiple leaves and use them as sponges to extract water or honey from cavities15. The bundles of leaves are jointly mobile, but they form loose, amorphous aggregates of equivalent parts, and it is debatable whether, for the purpose of understanding tool-related cognition, it is useful to integrate them in the category of compound tools. How such tool technologies are discovered and adopted by individual members of different natural populations is an important but as yet largely unsolved problem. In particular, the relative roles of genetic predispositions, individual innovation and social transmission are not easy to establish, because in wild populations all those factors contribute and are confounded from the observer’s perspective. In captivity in contrast, the acquisition process of complex novel behaviors such as compound tools can be studied and manipulated under controlled conditions.

The best-known case of the invention of a compound tool outside humans was reported in captivity nearly a century ago by Koehler7. He reported that a captive chimpanzee (Sultan) discovered how to combine 2 pieces of bamboo so as to make a longer compound pole with which he could reach bananas placed outside his cage7. Koehler’s observation is of major significance for comparative cognition, because it seems reasonable to hypothesize that for an individual to invent an instrument by joining different objects into a novel structure, anticipating its emergent properties (affordances) the individual must be capable of some form of cognitive modeling. Koehler, and many of his followers, have used the term insight in an explanatory role, but, in the absence of precise controlled experimentation and working definitions, such as used in human psychology (e.g.18,19), this does little more than labelling the observed sudden behavioural transition. His seminal observation, while frequently cited, has remained underexplored in later comparative cognitive research (See10 and13 for exceptions), and has not yet been extended to taxa beyond primates.

We were inspired by Koehler’s study to investigate compound tool inventions, but in the absence of social inputs, previous experience with similar tasks, or reinforcing feedback, in New Caledonian crows, a species notable for exhibiting multiple flexible and species-specific tool-related abilities20,21,22,23,24. Laboratory experiments have shown that these crows possess a capacity to innovate25,26,27 and to solve novel physical problems with sensitivity to at least some causal physical interactions between objects28,29. Given what is known about this species, it seems a strong candidate to explore its competence for compound tool making. Should this be successful, it would provide a novel and independently evolved biological model30 to help understanding the associated cognition.

The present study presented 8 wild-caught New Caledonian crows (NCC hereafter), with a problem solving task they had never encountered before. The study includes 5 phases: (i) set-up familiarization, (ii) first construction test, (iii) transfer tests (involving 2 modifications of the task), (iv) need discrimination test and (v) second construction test. In the set-up familiarization phase the subjects were presented with a novel setup, i.e. food placed in a track inside a transparent box, and wooden doweling pieces of sufficient length for them to push the reward along this track and out of the box (Figs 1 and S1). After the crows had successfully extracted the food, the first construction test followed, where the long dowels were replaced by 2 novel kinds of cylindrical elements, all too short to reach the food. These elements were potentially combinable, one kind being hollow (1 ml syringe barrels) and the other solid and thinner (syringe plungers or short wooden dowels; see methods and Supplementary Information, SI hereafter, for further details). They were presented both on holders and scattered on the ground (Figs 1 and S2). Subjects were allowed up to 6 attempts (of max. 12 min each) to retrieve the food. Successful birds entered into the third phase, that tested whether the production of compound tools depended on a particular shape and configuration of the combinable elements. This phase involved 2 transfer tests. In the first, subjects were exposed to novel materials; pipe cleaners were provided in addition to wooden dowels and the syringe barrels were replaced by drinking straws presented in a novel position, as well as loosely on the ground (Fig. S3). In the second, the ergonomic difficulty was increased by presenting all the straws and dowels solely on the floor (Fig. S4).

Figure 1 Experimental setup in construction test 1. Upper panels: test box without (A) and with (B) front cover. Notice the food track and side opening in A, and the narrow slot for tool insertion in the front in B. (C) Presentation of tool components. Some details of scale modified for presentation (see SI for details). Full size image

The fourth phase was designed to test whether the birds constructed tools in a goal-directed manner, driven by the need for compound tools, or simply because the action of combining was rewarding by itself. To this effect a ‘close track’ was added, from which a food target could be retrieved with uncombined elements (Fig. S5), and alternated between this situation and the original track, now called ‘distant’. After familiarization with both short and long tools (uncombinable dowels) simultaneously available in the presence of either of the tracks (see SI for details), we provided only short, combinable, tool elements, and positioned the food half of the trials in the distant and half in the close track, in pseudo-random order. Finally, in the fifth phase, the flexibility and goal-directedness of the behavior was examined by further challenging the birds that had succeeded in making stable 2-elements compound tools and extracting food with them in all previous phases. Now they were supplied with even shorter tool elements, so that 2-component tools were insufficient to extract the food (SI; Fig. S6) making an additional recursive combination necessary.