Proximate mechanisms of detecting nut properties in a wild population of Mexican Jays (Aphelocoma ultramarina)

In contrast to extensive research on optimal foraging in birds, the proximate mechanisms by which birds estimate the properties of nuts or seeds have not been well studied. Using slow-motion video-recording and experiments with modified peanuts presented to birds in their natural habitat, we explored these issues in a wild population of the Mexican Jay (Aphelocoma ultramarina). Jays evaluated each peanut by performing fast movements of the head combined with additional fast movements of the beak, which may open and subsequently close producing sound at the moment of hitting the shell. These movements seemed to provide Jays with additional sensory information that led to a more strict discrimination against non-preferred peanuts. We presented Jays with two types of peanuts that looked similar but differed in weight and found that, after handling the nuts, Jays consistently preferred the heavier nuts. In another experiment, the visually larger nuts with atypically lower mass (due to experimental alteration) were picked up easily but subsequently were rejected during handling, while the smaller peanuts with the weight typical for the size were easily accepted leading to the preferences for nuts with higher nutmeat density. This indicates that birds may have a concept of how much a nut of a given size should weigh, or alternatively that simple correlation between density of nut content and the properties of sound produced during handling lead to the ability of choosing denser nuts. We discuss further experimental studies that may bring more understanding of the proximate mechanisms of nut content assessment by birds.