He doesn't want the whole sheep, but only the kidney-fat.
This restricted taste makes him an expensive bird to support.

To get the fat he drives his beak in and rips it out; the wound is mortal.

This parrot furnishes a notable example of evolution brought about by changed conditions.

When the sheep culture was introduced, it presently brought famine to the parrot by exterminating a kind of grub which had always thitherto been the parrot's diet.

The miseries of hunger made the bird willing to eat raw flesh, since it could get no other food, and it began to pick remnants of meat from sheep skins hung out on the fences to dry.
It soon came to prefer sheep meat to any other food, and by and by it came to prefer the kidney-fat to any other detail of the sheep.