Piagetian liquid overconservation was investigated in four grey parrots (Psittacus erithacus). Birds tracked the larger of two quantities that had undergone various manipulations. Experiment 1 involved controls to ensure birds could track movement of the quantities, including direct and diagonal cross-transfers. All birds succeeded. In Experiment 2, different amounts in the same transparent or opaque containers were transferred into containers rigged such that amounts then looked equal. All birds chose the larger amounts after transformation when initial cups were transparent, but were random or had consistent side preferences when initial cups were opaque (thus obscuring quantity differences), showing that they used inferential, not perceptual, information, and that no extraneous cues existed. In Experiment 3, two birds saw different amounts from same-sized transparent or opaque containers transferred to containers of different sizes, rigged such that resultant amounts appeared to fill both cups and in which lesser amounts appeared greater on some trials. The older bird demonstrated full use of inferential abilities; he succeeded in all tasks when initial cups were transparent but, again, had consistent side preferences with initially opaque cups. The younger succeeded in the direct transfers with transparent initial cups but was random or showed a side preference on diagonal-transfer tasks and on all the tasks with opaque initial cups. However, she had no preference for the cup that appeared fuller, suggesting she did not use perceptual cues. Overall, grey parrots appear to understand the constancy of liquids undergoing physical transformation. (PsycInfo Database Record (c) 2020 APA, all rights reserved).