From a Livescience article:
New Caledonian crows actually do understand how to make water displacement work to their advantage, experiments showed. The results suggest that the birds are, at least in some respects, as smart as first-graders, according to the study.
“The ability to detect and respond to relevant causal properties demonstrated here, is striking — in spite of its limits — and rivals that of 5- to 7-year-old children,” the researchers of the new study wrote.
It seems that the more we study animals, the more we find that many have an intelligence much greater than initially estimated.