While humans are able to understand much about causality, it is unclear to what extent non-human animals can do the same. The Aesop's Fable paradigm requires an animal to drop stones into a water-filled tube to bring a floating food reward within reach. Rook, Eurasian jay, and New Caledonian crow performances are similar to those of children under seven years of age when solving this task. However, we know very little about the cognition underpinning these birds' performances. Here, we address several limitations of previous Aesop's Fable studies to gain insight into the causal cognition of New Caledonian crows. Our results provide the first evidence that any non-human animal can solve the U-tube task and can discriminate between water-filled tubes of different volumes. However, our results do not provide support for the hypothesis that these crows can infer the presence of a hidden causal mechanism. They also call into question previous object-discrimination performances. The methodologies outlined here should allow for more powerful comparisons between humans and other animal species and thus help us to determine which aspects of causal cognition are distinct to humans.

For many years nest building in birds has been considered a remarkable behaviour. Perhaps just as remarkable is the public and scholarly consensus that bird nests are achieved by instinct alone. Here we take the opportunity to review nearly 150 years of observational and experimental data on avian nest building. As a result we find that instinct alone is insufficient to explain the data: birds use information they gather themselves and from other individuals to make nest-building decisions. Importantly, these data confirm that learning plays a significant role in a variety of nest-building decisions. We outline, then, the multiplicity of ways in which learning (e.g., imprinting, associative learning, social learning) might act to affect nest building and how these might help to explain the diversity both of nest-building behaviour and in the resulting structure. As a consequence, we contend that nest building is a much under-investigated behaviour that holds promise both for determining a variety of roles for learning in that behaviour as well as a new model system for examining brain-behaviour relationships. Keywords: nest building, learning, cognition, comparative cognition, birds IntroductionThe notion that learning might be involved in nest building was not lost on inquiring minds in the 19th century, including that of Alfred Russell Wallace . He may have been the first to argue that nest building in birds was not due entirely to instinct: "[t]his point [. . .] is always assumed without proof, and even against proof, for what facts there are, are opposed to it" (Wallace, 1867). Ironically, despite the passing of nearly 150 years, Wallace's statement is as relevant today with regard to both the popular and scientific opinion as it was in his time. Indeed, at present nest building in birds is a behaviour considered to reflect nothing more than genes (Bluff, Weir, Rutz, Wimpenny, & Kacelnik, 2007;Hansell & Ruxton, 2008;Raby & Clayton, 2009;Seed & Byrne, 2010;Zentall, 2006). This view, however, continues to be based largely on untested assumptions, as there are very few data on how birds 'know' what type of nest to build.Helpfully, there are other aspects of birds' nest building that are quite well described. Indeed, several excellent bodies of work provide a broad overview and thorough discussion of this key component of avian reproductive biology (e.g., Collias & Collias, 1984;Deeming & Reynolds, 2015;Hansell, 2000). In brief, there are considerable data on the inter-and intraspecific variation in nest-site selection, composition, morphology, and building techniques. This wealth of data reveal an abundance of diversity in all these features of building: (a) birds build nests in an extraordinary range of different sites (Hansell, 2000); (b) where the individual builders of most species are known, nest building is not necessarily restricted to one of the sexes and contribution by one or both partners varies considerably from species to species (Collias & Collias, 1984;Hansell, 2000); and (c) nest material compos...

New Caledonian crows make and use tools, and tool types vary over geographic landscapes. Social learning may explain the variation in tool design, but it is unknown to what degree social learning accounts for the maintenance of these designs. Indeed, little is known about the mechanisms these crows use to obtain information from others, despite the question's importance in understanding whether tool behavior is transmitted via social, genetic, or environmental means. For social transmission to account for tool-type variation, copying must utilize a mechanism that is action specific (e.g., pushing left vs. right) as well as context specific (e.g., pushing a particular object vs. any object). To determine whether crows can copy a demonstrator's actions as well as the contexts in which they occur, we conducted a diffusion experiment using a novel foraging task. We used a nontool task to eliminate any confounds introduced by individual differences in their prior tool experience. Two groups had demonstrators (trained in isolation on different options of a four-option task, including a two-action option) and one group did not. We found that crows socially learn about context: After observers see a demonstrator interact with the task, they are more likely to interact with the same parts of the task. In contrast, observers did not copy the demonstrator's specific actions. Our results suggest it is unlikely that observing tool-making behavior transmits tool types. We suggest it is possible that tool types are transmitted when crows copy the physical form of the tools they encounter.

One source of public information may be the enduring products of others’ behaviour, such as discarded tools or vacated nests. Here, we examined whether observation of a nest affects the material captive zebra finch males prefer when they construct their first nest. It does: for first-time nest construction, males that viewed only an empty cage preferred the colour of material each initially favoured but those males that had observed a pre-built nest of material of their non-preferred colour lost their material-colour preference altogether. Additionally, half of the males that viewed a nest were tested in an environment (the laboratory) different to that in which they were reared (an outdoor aviary). We had expected the aviary-reared (versus laboratory-reared) males would be more uncertain, and thus more likely to select material for their first nest that matched in colour to the colour of the ‘demonstrated’ nest—but this was not the case. The aviary-reared males did, however, tend to touch first the demonstrated colour of material more than did the laboratory-reared males. Together these results show that both observation of a nest and a change in environment can influence the material choices of novice builders. For naïve animal builders, then, construction artefacts can be information resources for learning about potential construction material. Electronic supplementary material The online version of this article (10.1007/s10071-019-01240-x) contains supplementary material, which is available to authorized users.

It is highly difficult to pinpoint what is going through an animal’s mind when it appears to solve a problem by ‘insight’. Here, we searched for an information processing error during the emergence of seemingly insightful stone dropping in New Caledonian crows. We presented these birds with the platform apparatus, where a heavy object needs to be dropped down a tube and onto a platform in order to trigger the release of food. Our results show New Caledonian crows exhibit a weight inattention error: they do not attend to the weight of an object when innovating stone dropping. This suggests that these crows do not use an understanding of force when solving the platform task in a seemingly insightful manner. Our findings showcase the power of the signature-testing approach, where experiments search for information processing biases, errors and limits, in order to make strong inferences about the functioning of animal minds.