Two New Caledonian crows exhibited a novel tool behaviour not previously reported for any non-human species (cf. Bentley-Condit and Smith 2010; McGrew 2013; Shumaker et al. 2011): inserting a stick into an object and using the stick to transport it. The behaviour clearly differs from the contain mode, defined as placing “fluids or objects into or on top of another object (the tool) to control and/or transport them” (Shumaker et al. 2011, p. 14). Whereas containers can carry fluids or assemblages of very small objects, the tool reported here did not contain the object and transported only a single object. Moreover, control over the object did not appear to determine the behaviour; in most cases, the target object could be moved more effectively using the beak alone.
It is possible that the crows perceived the objects as potentially harmful and that such risk was mitigated by use of a tool. New Caledonian crows use stick tools to explore novel objects (Wimpenny et al. 2011) and avoid possible risks (Taylor et al. 2012). However, such explanations seem insufficient for our findings. In all cases, the crows had touched the objects with their beaks at least once before, so the objects were not truly novel. The crows also showed no neophobic reaction towards the objects and readily interacted with them. Neither does food search explain the behaviour. The objects in these observations had never been associated with food, and foraging techniques in the wild differ notably from our observations.
A possible functional explanation for insert-and-transport tool use is the simultaneous transport of both a tool and an object (or food item), which could be adaptive in the wild. New Caledonian crows often secure their tools while foraging, especially at greater heights where suitable tools become scarce (Klump et al. 2015). Although our experiment only involved non-food items, crows might target food as well: notably food with an opening that is either not immediately consumed or too large to be handled easily, such as half-opened seashells or large snails.
Subjects did not always transport the tool and object very far in our captive setting. There was also no clear reason why they would transport the object in the first place. Sometimes they brought their tool from another room, which was unnecessary if the goal was to transport the object, given that they could carry the object in their beak. In most cases, using a tool was less effective for transportation: it took time to insert the tool correctly and the tool-object combination was heavier. Given those time and energy costs, why would the crows use tools when they did not need to?
One case (Observation 2) indicated potential purposefulness of this behaviour; the subject struggled with grasping a large wooden ball and then successfully transported it with a stick. This indicates another possible function of this tool-use mode, namely control over unwieldy objects. The beak morphology of New Caledonian crows facilitates stick manipulation (Matsui et al. 2016; Troscianko et al. 2012), but it constrains handling many other objects. In the other observations, insert-and-transport tool use is perhaps best explained as a form of exploration or play because it was performed voluntarily in a low-stress setting without clear immediate benefit or purpose (Burghardt 2005), at least in the captive setting in which we have detected it. Playful stick handling develops into tool use in juveniles, even in the absence of demonstrators, which suggests it is an inherited action pattern (Kenward et al. 2006).
Insert-and-transport is a novel tool-use mode in animals as it differs notably from previously described modes. Our observations could be innovations originating in play and development without the purpose of transporting objects. Further research is needed to investigate whether it is a species-typical behaviour that might be adaptive, and controlled studies in captivity should establish the extent to which New Caledonian crows can apply this behaviour flexibly for purposes of transport.