Let's Learn About Macadamia Nuts

The macadamia nut has an extremely hard shell, but can be cracked using a blunt instrument, such as a hammer or rock applied with some force to the nut sitting in a concave surface, or a custom made macadamia nutcracker can be used. Macadamia nuts are often fed to Hyacinth Macaws in captivity. These large parrots are one of the few animals, aside from humans, capable of cracking and shelling the nut. Nuts of the "Arkin Papershell" variety crack open more readily.

Now you know.

http://en.wikipedia.org/wiki/Macadamia