Cheap and designed for the chewer. This is the small. I stuff a peanut in the hole. Half out, easy access, and half in for the more experienced forager.
Dog Kong is a GREAT toy! I've got a large Kong for my Macaw who LOVES banging it around, snuggling under it, AND chewing it.
With these Kongs, however, you have to make extra sure you use only very little rope as you don't want your birds' head wrapped around it. With the rope on one end and the Kong at the end of the rope acting as weight, it would be like a noose.