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.
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 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.
What’s 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.
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.