{"fact":"One tricky hiding task, involving moving a treat behind several screens and leaving it behind the first one, stumped all the crows in the study.","context":"The last and final task of Stage 6, task 15, was never passed by any bird. The crows repeatedly searched behind the screen where the hand had disappeared last.","source":"Almut Hoffmann; Vanessa Rüttler; Andreas Nieder, Ontogeny of object permanence and object tracking in the carrion crow, Corvus corone, 10.1016/j.anbehav.2011.05.012","index":179}