【正确答案】A kind of birds that prey on shellfish, they swoop down from sky to catch the shellfish in the water,then fly to a certain height and drop the shellfish on rocks for them to crack open, the birds choose to drop the biggest shellfish every time to obtain the highest energy, also they fly up to 5 meters high to drop the shellfish. It's neither too high for the birds to waste energy, nor too low for the shellfish not to crack open.