Bengt Heinrich, writing in "Mind of the Raven", found their limit by tying bits of meat to strings hanging where they could reach the string. Some, not all, would grab the string with their beak and tuck it under one foot, and keep drawing it up until they got the meat. But none ever figured it out if the string was hung over a branch so they had to pull down to bring the prize up.
Bengt Heinrich, writing in "Mind of the Raven", found their limit by tying bits of meat to strings hanging where they could reach the string. Some, not all, would grab the string with their beak and tuck it under one foot, and keep drawing it up until they got the meat. But none ever figured it out if the string was hung over a branch so they had to pull down to bring the prize up.