They have white plumage, with only the face and leg colours distinguishing the two species. They look plump and dove-like, but are believed to be similar to the ancestors of the modern gulls and terns.
They derive their English name from the horny sheath which partially covers the upper mandible of their stout bills.
The sheathbills are scavengers, but will take chicks and eggs as well as offal.
They lay 2 or 3 blotchy white eggs on the ground.