The extent to which non-humans understand their physical world is controversial, due to conceptual and empirical difficulties.We examine the evidence for physical understanding in the remarkable tool-oriented behaviour of New Caledonian crows, which make several types of tool in the wild and show prolific tool-related behaviour in captivity.We summarize our own research into the cognitive processes involved in tool behaviour in this species, and review comparable studies in other birds and primates.Our main laboratory findings are: tool-related behaviour emerges in juvenile crows that had no opportunity to learn from others; adult crows can make or select tools of the appropriate length or diameter for tasks; and one crow, at least, can bend and unbend novel material to match task requirements.Although these observations are striking, they do not prove that this species is capable of understanding physical causality, as one cannot exclude explanations based on inherited proclivities, associative learning, and generalisation.Despite this, we argue that the conventional mechanisms become less likely as such observations accumulate.We conclude that while no adequate, non-verbal test for understanding exists, continued work with New Caledonian crows will help us to ask the right questions.
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