The beaks of male and female huia are so different that the two sexes were originally described as two separate species. But what is the reason behind that difference? Natural History Curator Rodrigo Salvador takes us through recent research.
Huia, or Heteralocha acutirostris, were distributed through most (if not all) of the North Island of Aotearoa New Zealand. Their numbers declined fast after Polynesian settlement, to the point that by the time Europeans arrived the species was already restricted to the southern end of the North Island.
Huia had the most extreme male/female difference in beak shape known in all birds. Accordingly, many naturalists at the time tried to figure out why.
Most believed that a huia pair would feed cooperatively, with each sex complementing the diet of its mate with the different types of prey their beaks allowed them to catch. That idea was – and still is – widespread in popular accounts of the huia, but it is a misinterpretation of 19th-century reports.
Others held that the beak shape was a secondary sexual character used in courtship, like a peacock’s tail. But female huia had the extreme beak shape, not the male. So, that doesn’t quite fit with what is known about sexual selection in birds.
A third idea was that the beak shapes allowed huia to feed on different prey to avoid competition for food within the pair. That is the most likely idea and the one we wanted to test.
But given that the huia is extinct and all, it is very difficult to get an actual answer. So, we turned to chemistry.
The “magic” of isotopes
Chemical elements typically come in more than one form. Broadly speaking, most atoms of any given chemical element will have the same weight – but a few atoms can be heavier.
For instance, carbon has three naturally-occurring forms, with molecular weights of 12, 13 and 14, and each of these forms is called an isotope. The carbon isotope of weight 12 (or 12C) is by far the most common, while 13C is quite uncommon and 14C is very rare (not to mention radioactive).
Everything is made up of chemical elements, so we can measure the proportions of isotopes of a given element in all sorts of material. We can, for instance, measure the isotopes in an animal’s body. But why would we want to do that?
The isotopes contained within our bodies reflect what we have eaten and also the environment where we live and where our food was grown. We are what we eat, so to speak.
As such, we can use the information of isotopes to extract ecological data. This is a very useful tool for gaining knowledge on species that are difficult to observe in the wild. Or impossible to, like, say, an extinct New Zealand bird.
So, we measured the isotopes of carbon and nitrogen in huia specimens. Carbon and nitrogen are good sources of information about what the animals were eating. So, we could use that as a way to tell if male and female huia were feeding on different things.
We used bits of feathers from taxidermied museum specimens for measuring the isotope content.
Our specimens here at Te Papa weren’t near enough, unfortunately. So, we obtained additional huia feathers from museums all around the world: Cardiff, Christchurch, Copenhagen, Helsinki, Invercargill, Paris, Pittsburgh, Stockholm, and Vienna.
Our data showed that the isotope signatures of male and female huia were not as distinct as we would have expected if they were feeding on completely different things. The isotope signatures were different, but they had a large overlap.
We also measured isotopes from specific amino acids that make up the proteins in the huia’s feathers. That extra data allowed us to calculate the trophic position of male and female huia. You can think of the trophic position as the level an animal occupies in the food chain.
Even so, male and female huia occupy identical trophic positions. Females, however, showed less variability in their trophic position.
So, are male and female huia different or not?
We interpreted our results as a partial dietary segregation between males and females.
While both males and females fed on herbivorous insects and on plant material (berries and seeds), the females would have a more specialised diet, feeding on a smaller variety of food items when compared to the males.
The unique features of the huia make them a good model to tackle questions of sexual segregation in diet. It was, after all, the avian example chosen by Darwin to exemplify sex-related differences driven by natural selection rather than by sexual selection.
Another important aspect of our research is to show the value of museum specimens: it’s possible to study aspects of behaviour and ecology even in extinct and poorly known species through those specimens. This was only possible thanks to the abundance of curated bird specimens being kept and cared for in museums collections worldwide.
This research was published in the scientific journal Oecologia: Extreme bill dimorphism leads to different but overlapping isotopic niches and similar trophic positions in sexes of the charismatic extinct huia. It was led by former Te Papa researcher Barbara Tomotani (now at NIOO-KNAW), in collaboration with Te Papa curators Rodrigo Salvador and Colin Miskelly, former Te Papa head of science Susan Waugh, and colleagues from NIWA, Muséum national d’Histoire naturelle, and La Rochelle Université.
The research was funded by the Dutch Research Council (NWO); part of the isotope analysis was funded by Birds New Zealand.