Screechers are giant, red and black birds found on large islands. There is one screecher on every large island which means you will have to visit at least 50 large islands to get Expert Screecher Hunter.
Screechers will attack you by picking up boulders from the island and dropping them on your head. It will only take a couple of boulders to kill you so you have to be paying close attention to where the screecher is at all times.
It is possible to stab screechers with your spear when they land to pick up a boulder, however, this will cause you to have to do a lot of running around the island and it will be hard to get to where the screecher has landed in time to stab them. Attacking the screecher with your spear is also dangerous because the screecher can drop it's boulder on you immediately as it takes off from the ground, not giving you enough time to avoid taking damage.
The easier way to kill screechers is to get to a high point on the island with your bow and shoot down on the screecher as they fly up to you. It will take 7 shots with metal arrows on normal difficulty to kill a screecher.