A 10m-deep death pit in Martinborough holding thousands of bones from unlucky flightless birds has been dubbed the richest site in New Zealand for certain species, including moa.

"It's just packed with bones under your feet," said Te Papa's curator of vertebrates Alan Tennyson, who paid a visit to the small cave earlier this year.

"The cave is relatively famous, I suppose, in the bird palaeontology world of New Zealand, because it has produced a lot of bones and it's still got bones in it.

The pit is easy to spot now, but thousands of years ago would have been hidden by thick forest. Photo / Te Papa
The pit is easy to spot now, but thousands of years ago would have been hidden by thick forest. Photo / Te Papa

"This particular cave is incredibly rich ... thousands of bones already found, and there's probably thousands more still sitting in there.

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"The funny thing is, it's really a tiny, little hole."

The entrance to the hole is only a few metres wide, but it's easy enough to spot now the area is clear. Thousands of years ago when the land was covered in thick forest, hapless birds would fall in and become trapped among the skeletons.

"On the surface there are modern animals still there," Tennyson said.

"A couple of dead sheep there. There are still animals falling into that hole."

Alan Tennyson brought a small amount of samples back after his expedition earlier this year. Photo / Te Papa
Alan Tennyson brought a small amount of samples back after his expedition earlier this year. Photo / Te Papa

The hole was first discovered in 1914 by a deer stalker who reported finding moa bones, and Te Papa's forerunner, Dominion Museum, launched an expedition in 1920.

The diverse range of bones include those of kakapo, kiwi, North Island takahe, weka, and the extinct adzebill, a bird that used to be about 80cm tall.

Many of the bones, some of which come from other animals such as the tuatara or some species of frogs, have been excavated and are held at Te Papa Museum now.

"It's not a unique site. The main ways that fossil bird bones are found are in sand dunes, swamp deposits, and caves. There are a lot of caves in New Zealand, obviously, and quite a few of them have produced bones. This cave is just particularly rich, probably the richest site for some of the species that we now have excavated over more than 100 years."

Tuatara jawbones have been found among the skeletons. Photo / Te Papa
Tuatara jawbones have been found among the skeletons. Photo / Te Papa

Any further expeditions to the cave, which is known as a pitfall trap, are on hold for now. Tennyson said there were still many bones to be sorted, and the cave was a good place to preserve the rest until the museum had the time and resources to excavate and sort more.

There was some argument for leaving bones where they lay, with many cavers preferring them to remain put for "aesthetic reasons".

"There's some benefits in leaving bones in situ because we don't know what future studies might be thought of," Tennyson said.

"The bones in this particular cave are actually a really good source of DNA."

There was not much room to move around at the bottom of the cave, Alan Tennyson said. Photo / Te Papa
There was not much room to move around at the bottom of the cave, Alan Tennyson said. Photo / Te Papa

The most common type of bone found in the cave is that of the extinct Finsch's duck, quickly recognisable by its beak. The trap is also the country's richest source of adzebill bones.

Because the hole is a pitfall, it is dominated by the remains of flightless birds that "blundered along" and fell in, unable to fly back out again.

"Once they fall down the hole there's no way of getting out."

The "key thing" about the site was that it showed researchers where certain species of bird used to be before their extinction.

"We can do research on them, looking at their relationships for example, based on the DNA. It's an amazing resource of material."