Testing the causal understanding of water displacement by kea (Nestor notabilis)

Abstract

Abstract Rooks, Eurasian jays and NC crows have already solved the Aesop’s fable paradigm, using stones to raise a water level to get a previously out of reach reward. Here we test kea parrots on their understanding of water displacement. In Experiment 1, they preferentially chose the water-filled tube over two solid substrates (sand, rocks) within 5 trials. In Experiment 2, they generalised to novel coloured substrates in the first trial. In Experiment 3, confronted with two water-filled tubes, one of which was made unsolvable by preventing the water from rising, none of the kea managed to solve the task. This suggests that their performance in Experiments 1 and 2 was reward association with the liquid-filled tube, without attendance to changes in water level. These results therefore provide no evidence of causal understanding of water displacement and highlight that such an understanding is not required for solving Aesop’s Fable.