These birds are waders, and are normally found on the edges of water, by lakes or the sea. They have a very loud and distinctive piping call in flight, a continuous 'k-peep k-peep k-peep'. You will often hear an oystercatcher before you see it. Their visible appearance is easily identified - black and white with a long bright orange bill (a magpie eating a carrot).
They feed by probing the mud, or poking around in rock pools. They can open shellfish with their strong bills.

Below, you can hear the peep-peep call of the Oystercatcher here (drowned out a little, by the seaside sounds)...