Wild Carrion Crows Choose Walnuts Based on Visual and Weight Cues

Assessment of the physical properties of concealed food items provides insight into the perceptual and cognitive mechanisms of animals. This study investigated the walnut selection behavior of wild carrion crows (Corvus corone) through two field experiments. In Experiment 1 (visual size preference), we presented walnuts with distinct visual sizes. Crows significantly preferred the larger walnut in both their first contact and final retrieval. In Experiment 2 (internal content preference), we presented walnuts of similar size but with different internal contents: filled or empty. While there was no preference for the first contact, crows significantly selected the filled walnut for final retrieval. The final selection was significantly influenced by the crows' handling behavior. In Experiment 2, the probability of switching was significantly higher when the first contact walnut was empty. Pre-retrieval lifting behavior also played a distinct role in each experiment. In Experiment 1, lifting either the larger or smaller walnut increased the probability of it being ultimately retrieved, regardless of its size. In contrast, in Experiment 2, the number of pre-retrieval lifts of the filled walnut significantly predicted its correct selection, whereas lifting the empty walnut had no such reinforcing effect on its retrieval. In both experiments, the probability of selecting the higher-quality option increased with the log-transformed weight ratio between the two walnuts, consistent with psychophysical principles of proportional weight discrimination. These findings demonstrate that wild crows flexibly integrate visual and weight information, utilizing active haptic sampling to compensate for unreliable visual cues and optimize foraging choices in a natural setting.