The Pacific Gull has a curious habit of dropping turban shells and other molluscs, especially limpets and mussels, from a height onto a rock in order to break them open. On some islands and headlands, this has resulted in the formation of 'middens' of shell fragments that could be mistaken for ancient natural beach deposits. (extract from Birds of Australia: http://birdsinbackyards.net/finder/display.cfm?id=249)