Duck Tales: unveiling the ecology of the extinct Chatham Island duck

Duck Tales: unveiling the ecology of the extinct Chatham Island duck

Curators Rodrigo Salvador and Alan Tennyson, working with colleagues from GNS Science and the Netherlands Institute of Ecology (NIOO-KNAW), used chemical analysis on the sub-fossil bones of this extinct duck to determine how it lived. Here they describe what they found.

The Chatham Islands, despite their limited size, had a diverse fauna of waterfowl made up of nine species. Four of them are now extinct, among which is the Chatham Island duck, Anas chathamica.

We know about this species through sub-fossil bones that can be found on the island. The species was still alive when the first humans arrived on the Chathams, but this duck was only recognised as a distinct species in 1955.

The Chatham Island duck has been dubbed New Zealand’s ‘most enigmatic duck’. It is related to the brown teal, but it’s much larger – in fact, it’s the largest bird in the globally widespread genus Anas. It was also flightless.

Bonus features: Distinctive morphology

Being a flightless island-living bird is a common thing, especially in New Zealand. But the Chatham Island duck also had two extra features that made it quite unusual: carpal weaponry and salt glands.

The name carpal weaponry is because it’s found on a wing bone called carpometacarpus, which is the fused wrist and knuckle bones. It’s a large structure protruding from the bone that the birds would have used in aggressive displays and combat. This was likely related to territorial disputes, similar to what is known for steamer ducks.

A large white bone with a split in the middle on a black background with a white line under it and the text 5 cm.
The knob-like structure on the top right is the so-called carpal weaponry’.
Chatham Island Duck, (Pachyanas chathamica), collected 20 January 1993, Tahatika Creek, New Zealand. CC BY 4.0. Photo by Jean-Claude Stahl. Te Papa (S.032634)

Salt glands are organs whose purpose is to excrete salt without losing water, thus keeping the animal hydrated. Sea birds typically have well-developed salt glands. The Chatham Island duck also had quite developed salt glands. They can be seen on the skull as large impressions located above the eye socket.

The skull of a duck sitting horizontally on a black background. There's a red arrow pointing to an eye socket and a white line with the text 2cm underneath in the bottom left corner
Skull of a Chatham Island duck with the red arrow showing the impressions of the salt glands immediately above the eye socket. Chatham Island Duck, (Pachyanas chathamica), collected 19 February 1991, Maunganui Beach, Chatham Island, New Zealand. CC BY 4.0. Photo by Jean-Claude Stahl. Te Papa (S.029475)

The fact that this duck has such large glands led previous researchers to hypothesise that this species had a marine lifestyle. However, a hypothesis like that is difficult to test when a species is already extinct.

Whats in the mix – Chemical analyses

So we partnered up with colleagues from GNS Science and the NIOO-KNAW to try and settle this matter. To do that, we analysed the chemical composition of the bones of Chatham Island ducks – and compared it to other New Zealand species of waterfowl.

More specifically, we looked into the proportion of heavy-to-light isotopes of carbon and nitrogen.

The proportion of isotopes found in an animal’s body depends on its diet and the environment where it lived. So structures such as bones act like an archive of the animal’s life and can be used to reconstruct the ecology of a species.

Needless to say, this approach is particularly useful for extinct species.

Nitrogen isotopes are related to the types of food the animal ate. Our results show that Chatham Island ducks had a diet rich in marine invertebrates.

Meanwhile, carbon isotopes are more related to the animal’s habitat. For this species, our results indicate that they inhabited the brackish Te Whanga Lagoon as well as more fully marine environments along the island’s coast.

An image of two x-y axis graphs with different-coloured dots showing the values for each duck. Graph A is nitrogen values and graph B is carbon isotope values .
Graphs showing the (A) nitrogen and (B) carbon isotope values for NZ waterfowl species, with mean and standard error bars. Note that the brown teal (A. chlorotis) and grey duck (A. superciliosa) are split into island (Is) and mainland (NZ) populations. Image adapted from Rodrigo Salvador et al. (2021)

Our research shows the importance that the marine environment played in the life of Chatham Island ducks.

It is also a good reminder of the key role natural history collections can have in ecological studies. Our research article was just published in the Emu, the journal of austral ornithology.

Further reading

You can learn more about other extinct Aotearoa New Zealand waterfowl in NZ Birds Online. You can also check the book Extinct Birds of New Zealand from Te Papa Press.

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