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The trap tube is a classic test of causal reasoning abilities in animals in the physical domain. Recently, a modified version of this task improved its diagnostic capacity and allowed testing of non-tool-using animals. We used this modified two-trap tube task to compare the cognition of two Darwin’s finch species: the woodpecker finch, Cactospiza pallida, a tool-using species, and the small tree finch, Camarhynchus parvulus, a closely related non-tool-using species. Not all woodpecker finches use tools in nature, and we therefore also tested non-tool-using individuals to assess the effect of experience on trap tube performance. No small tree finches and only two non-tool-using woodpecker finches solved the initial task which was operated using a pre-inserted piston. One tool-using woodpecker finch solved the task when allowed to use its own tool instead of the pre-inserted piston. The fact that none of these subjects transferred their knowledge when the features of the task changed, suggests that in this species, neither experience using tools nor the genetic composition of a tool-user are associated with the general physical cognitive skills required to solve the trap tube task.

Darwin's finches are a classic example of adaptive radiation. The ecological diversity of the Galápagos in part explains that radiation, but the fact that other founder species did not radiate suggests that other factors are also important. One hypothesis attempting to identify the extra factor is the flexible stem hypothesis, connecting individual adaptability to species richness. According to this hypothesis, the ancestral finches were flexible and therefore able to adapt to the new and harsh environment they encountered by exploiting new food types and developing new foraging techniques. Phenotypic variation was initially mediated by learning, but genetic accommodation entrenched differences and supplemented them with morphological adaptations. This process subsequently led to diversification and speciation of the Darwin's finches. Their current behaviour is consistent with this hypothesis as these birds use unusual resources by extraordinary means. In this paper, we identify cognitive capacities on which flexibility and innovation depend. The flexible stem hypothesis predicts that we will find high levels of these capacities in all species of Darwin's finches (not just those using innovative techniques). Here, we test that prediction, and find that while most of our data are in line with the flexible stem hypothesis, some are in tension with it.

Behavioural flexibility is thought to be a major factor in evolution. It may facilitate the discovery and exploitation of new resources, which in turn may expose populations to novel selective forces and facilitate adaptive radiation. Darwin's finches are a textbook example of adaptive radiation. They are fast learners and show a range of unusual foraging techniques, probably as a result of their flexibility. In this study we aimed to test whether variability of the environment is correlated with flexibility. We compared woodpecker finches from a dry area (hereafter, Arid Zone), where food availability is variable, with individuals from a cloud forest (hereafter, Scalesia zone) where food abundance is stable. As parameters for flexibility, we measured neophilia and neophobia, which are two aspects of reaction to novelty, reversal learning and problem-solving. We found no differences in performance on a problem-solving task but, in line with our prediction, individuals from the Arid Zone were significantly faster reversal learners and more neophilic than their conspecifics from the Scalesia zone. The latter result supports the notion that environmental variability drives flexibility. In contrast to our prediction, Arid Zone birds were even more neophobic than birds from the Scalesia Zone. The latter result could be the consequence of differences in predation pressure between the two vegetation zones.