Mummified gull/tern chick

[David Riddell]

I was out at Waikorea Beach (north-west Waikato) yesterday doing a beach patrol and found an interesting range of birds washed up, including a short-tailed shearwater, 4 sooty shearwaters, a fluttering shearwater, a white-capped mollymawk and 2 common diving petrels. But I was a bit puzzled by the bird in the photo below. Looks like a gull or tern chick of some sort, obviously bred locally, though in the four years I've been beach-patrolling at Waikorea I've never seen any indication of gulls or terns breeding there. Black-backed gulls are by far the most abundant species on the beach, though there are always a few Caspian terns and from the size (c. 20 cm tip to tail) I'd suspect it's one of those two species. The bill looks very slender though since it's missing its sheath that could be misleading. Any advice from anyone with more experience of young gulls and terns would be appreciated. It was near the mouth of the Waikorea Stream where there is an extensive sand flat (a pair of variable oystercatchers has a nest with two eggs there at the moment) and an area of sand dunes behind the beach.