A Bird’s Eye View of Weight: Do Kea Rely on Visual Cues to Infer Weight?
Humans utilize an assortment of clues to determine the weight of an object, including but not limited to size, material, apparent density, and quantity. Previously believed to be a purely human capability, studies with species such as capuchins have shown that animals may also be capable of inferring weight from the visual qualities of an object . To date, however, this phenomenon remains relatively unexamined and few studies have explored the extent to which other species attend to the visual signals regarding the weight of an object. In this study, we aim to test whether kea (Nestor notabilis), known for their curiosity and physical problem-solving skills, can use the visual cues of size, material, apparent density, and quantity (i.e., how full a container is) to determine the weight of an object prior to directly interacting with it.
The kea were previously trained to drop either a light (16g) or heavy (116g) object into a box. In this study, the kea are presented with pairs of objects – one light and one heavy – from the four different categories and will be rewarded for dropping the object of their target weight into the box. We measure i) which object the birds first contact, ii) whether they switch after handling (and feeling the weight of) incorrect objects, and iii) which object the birds drop into the box.
The results from this study will provide insight into whether kea can use visual cues to infer weight and, if so, which visual cues they rely on. The findings of this study will help shed light on the physical cognition of this species and contribute to the conversation surrounding the capacity of non-human animals to have a theory of weight.
 B. L. De Moraes, A. Da Silva Souto, and N. Schiel, “Adaptability in stone tool use by Wild Capuchin Monkeys (sapajus libidinosus),” American Journal of Primatology, vol. 76, no. 10, pp. 967–977, 2014.