Studies: Crows count, chimps know when they're right - CNN

https://www.cnn.com/2015/06/10/us/chimpanzee-crow-intelligence-studies/index.html

I'd heard about crows counting how many hunters were behind a hunting blind, (mentioned in this article) but I wondered how they had measured the change in behavior when the crows knew the hunters had gone. These experiments seem more sophisticated than that at proving how crows and chimpanzees think. Metacognition..is knowing that you know something...They gave three chimps computerized memory tests and rewarded them with food for correct answers, but they built in some hitches to tease out signs of metacognition..The reward appeared at a place away from the test area, and when the chimps got an answer right, a sound alerted them that the food was going to appear -- with a slight delay -- at the distant spot. They had to hustle to get to the other station before the food popped out, or it would disappear, or go down the drain, so to speak.
But that alerting sound was delayed, too. The apes barely had enough of a heads up to make it to the reward station in time to catch the food before it vanished.
After a while, the chimps appeared to get a sense for when they got an answer right and when they didn't. And when they did, they often darted for the food station ahead of that tardy sound signal.
If the monkeys got an answer wrong, more often than not, they sat things out. After all, when you've bombed, why go to the trouble?.