The evolutionary origins and ecological context of tool use in New Caledonian crows

Extreme binocular vision and a straight bill facilitate tool use in New Caledonian crows

“…It is currently impossible to establish causal relationships, because the number of tool-using bird species is insufficient for broad comparative analyses, and the current lack of NC crow (sub-)fossils prevents dating of key evolutionary events 6 . In other words, it remains an open question whether ancestors of NC crows possessed morphological features that predisposed them to tool manufacture and use, or whether tool-use behaviour was expressed first due to unusual ecological opportunities, before exerting selection pressures that gradually shaped the species' visual system and bill morphology 6 .…”

“…Using their bills, these tropical birds craft complex tools from sticks, leaf edges and other materials, before inserting them into deadwood or vegetation to 'fish' for invertebrate prey 6 . In this study, we show that NC crows present at least two unusual morphological features that combine to facilitate their use of tools: extreme binocular vision and an uncharacteristically straight bill.…”

Preliminary observations of tool-processing behaviour in Hawaiian crowsCorvus hawaiiensis

“…Our preliminary observations have confirmed that ‘Alalā are capable of modifying and manufacturing tools in a variety of ways, paving the way for dedicated experiments that systematically chart the species’ natural tool-making repertoire, as well as its ontogenetic development [4,22–24]. Importantly, subjects need to be tested under conditions where they have access to a wide variety of naturalistic foraging tasks (including those routinely encountered by wild New Caledonian crows [2,3,11,12,16]), and different plant materials for tool manufacture, but not to sticks or other objects that could be used as tools without further processing (as was the case in our first study [4]). This work programme, which we have recently launched, borrows experimental designs we developed for assaying the tool-manufacture behaviour of wild-caught New Caledonian crows [8,25–27].…”

“…Only a handful of bird species habitually make and use foraging tools [1], amongst them the well-known New Caledonian crow Corvus moneduloides [2,3]. Recently, we discovered that a second tropical corvid – the Hawaiian crow or ‘Alalā C. hawaiiensis – is a highly skilled, natural tool user [4].…”

“…New Caledonian crows also frequently engage in bark flaking (43% of recorded bill interactions in a study using bird-borne miniature video cameras [11]), and exhibit a wide range of tool-processing behaviours [2,3,8,11–18], but to our knowledge, there have been no reports of bark tool use in the wild. Interestingly, the only observation to date of a wild American crow C. brachyrhynchos using a tool for (presumed) extractive foraging involved the use of a wooden splinter, which the bird had released from a fence post through pecking and pulling [19].…”