“We’ve got one,” calls Kristal Cain, disentangling a tītipounamu’s spindly legs from a mist net strung between trees. The bird is a male, a green jewel with pale grey underparts and a thin beak like the tip of a hypodermic needle, and he’s surprisingly warm. Cain can feel the life thrumming within him. He’s no heavier than a coin, and so tiny and spindly that it’s like holding a dandelion-seed head—he’s all long legs and round body.
Jessica Peart is already behind the camera, which is pointed at a black-velvet backdrop—a photobooth set up in the middle of the forest. Holding the bird delicately by his legs, almost as though he’s a pencil, Cain poses him in front of the black backdrop. He regards the pair of scientists calmly.
Peart leans over the camera with intense concentration and snaps two photos: one in visible light, the other with an ultraviolet filter. Once the bird has fulfilled his modelling duties, he’s released into the bush, his green plumage quickly melting into the foliage.
To our eyes, the Hawke’s Bay bush is a shifting palette of lush greens, rich browns and dappled sunlight. To a tītipounamu, it’s an extra-Technicolor world: hundreds of hues where we see just one. Foliage shimmers, each leaf distinct, while fungi and fruits fluoresce. Other birds, lit up and lustrous, flit through the bush in high definition, where humans perceive only a feathery blur.
The ability to see arises from colour-detecting light receptors (called cones) in the eyes. Humans have three types of cone, each specialised to detect different wavelengths of light: red, green and blue. Birds also have red, green and blue receptors, but they have a fourth cone type that can see violet or ultraviolet (UV). Special oil droplets on their cones filter the light more finely—meaning they see more shades of colour than we do. Birds also see more motion detail: although human and bird eyes both function like cameras, birds’ eyes are tuned to a faster shutter speed—they can perceive up to 100 light pulses per second, compared to the 50 per second that the human eye can perceive.
Since the accidental discovery of birds’ ultraviolet sight in the early 1970s, scientists have found puffin beaks reminiscent of glowsticks, blue tits with gleaming head caps, and parrot plumage gaudier than thought possible. In New Zealand, pōpokatea have ultraviolet heads and chests, and the colour is brighter on males than females. Other than the pōpokatea, though, no one has taken a good look at New Zealand’s oddball birds in UV—until now.
Birds see a world that our human brains can scarcely imagine—and we don’t know why, or what that world holds. We do know that ultraviolet sight may help them find food, as some plants and insects fluoresce in UV. But that doesn’t explain why birds’ feathers light up in ultraviolet as though they’re dressed for a rave.
Tītipounamu are also called riflemen because the male birds’ colour reminded someone of military dress, and their uniform does seem like perfect camouflage—except that it’s not the only colour they’re wearing. Peart’s ultraviolet photograph reveals patterning on the rifleman invisible to the human eye. He has silvery dots of eyeliner, ridges along the edge of his wing coverts and a luminous patch on his chest.
“The world that we see is not the world that all animals see,” says Cain, a biologist at the University of Auckland. Her lab is studying tītipounamu from all angles—song, genetics, vocal learning ability, sensory ability, behaviour and colour—because the birds are an evolutionary puzzle piece. They’re wrens, an ancient family older than songbirds, which means they could tell us how and why other birds evolved.
Ultraviolet colouration is a significant part of the tītipounamu’s mystery. How do the birds use it? What do UV patterns mean? Do they signal something to other birds?
“Lots of animals have communication channels we are oblivious to,” says Cain. “Just watch a dog sniffing at a post and you get a sense of what I mean. They are getting loads of information from those sniffs, information we cannot even imagine.
“Using [colour], birds can communicate with each other without other species having any idea what they are saying or even being aware they are communicating. They are communicating their sex, their quality, perhaps their dominance or parental care abilities, and doing it on a channel that most animals cannot detect. We don’t even notice it—that there’s all this other stuff.”
“All this other stuff” is now forcing scientists to go back to the drawing board in search of explanations for why birds are the way they are—their colour, size, shape, behaviour and diet. But this time, from a bird’s-eye view.
“Look at this hind claw,” says Sarah Withers, pointing to the curved talon extending from the back of the tītipounamu’s foot. “It’s crazy, right? It’s much more substantial in the females. She’s just beefier, full stop.”
The ornithology lab at Auckland Museum is a beige room filled with rolling cabinets and the hum of air conditioning. Behind the beige, there are hidden treasures. The cabinets are labelled ‘MOA’, ‘BONES’, and ‘EXTINCT’. There is a box with tiny peachy eggs laid on soft cotton wool, and, perched on a table, what we came for: a taxidermied bird mounted under a grand Victorian bell jar. Unlike the vibrant green male tītipounamu, the female is speckled brown and grey on her back, and she’s bigger and heavier. Her thick legs and grippy talons keep her sturdy on the steep surfaces of tree trunks, where she spends the most time foraging. “She also has a slightly upturned bill compared to the male, for probing under the bark,” says Withers, a postdoctoral researcher in Cain’s lab.
Just like the females’ long talons evolved to help them cling to branches, their colours are purposeful, too. Some birds have “cryptic” colours designed for camouflage, while bright colours are advertisements for potential mates. Browns, greens and mottled patterns like the female’s plumage may keep her hidden among bark, lichen and twiggy nests—but this assumption is based on how humans see birds, not how birds see birds.
“New Zealand species haven’t been the subject of much colour research,” says Withers.
“We have quite muted colouring in birds—we don’t have lots of brightly coloured birds like Australia does—ours are often brown and green. I want to challenge the idea that green means cryptic.”
Peart removes the tītipounamu from the bell jar and positions it beneath the UV-modified camera on a piece of black velvet. She has been precisely measuring the colours of the tītipounamu and the colours of their habitats using photography, and figuring out how closely they match, “from the bird’s colour perspective, not our perspective”.
It’s thought that males spend more time among the leafy foliage, while females tend to cling to the trunks of trees. And according to Peart’s master’s research—she has spent countless hours recording how long the birds spend on branches versus trunks versus leaves—this is true. “But in terms of the colours they’re spending time on—green leaves and moss, brown bark, and white lichen—there’s no difference,” says Peart.
This suggests that the male’s green is perhaps not driven by a need to blend in—he spends plenty of time contrasting with his surroundings, especially while incubating eggs on the brownish nest. Plus, wouldn’t his razzle-dazzle UV be the opposite of camouflage for the avian eye? For thousands of years, the tītipounamu’s main predators were other birds, such as ruru/morepork, and it’s possible that they, too, have ultraviolet sight. (We don’t know how sensitive to UV light New Zealand’s birds of prey are.)
Moreover, if the rifleman evolved to blend in, you might expect to find a variety of different rifleman uniforms to match the different habitats they live in—just like the military has a range of camouflage patterns to match desert, jungle, mountains and woodlands.
“Historically, they would have been everywhere: at the top of mountains and right down in the swamps and foraging on the ground,” says Cain. “But they don’t look terrifically different if you go from Rakiura to Cape Reinga. You would think that if they’re really adapted to be cryptic, you’d see different colours when they live in different places.”
Some populations have been geographically isolated for millions of years, and yet their appearance and behaviour have stayed the same.
“They’re like the crocodiles of birds,” says Cain. “They’re like, ‘This works, we’re not changing.’”
So, if the colour difference between males and females isn’t for disguise, what causes it? The way that tītipounamu make colours may offer a clue.
At the museum, Withers opens the first of three white boxes, laid out by the collections manager. Inside lie the delicate skins of riflemen collected in 1878 and 1934, wings and legs tucked and heads extended.
Wielding a set of tweezers, Withers leans in close to select a single feather to pluck from a skin. As part of her investigation into the colour green, she is zooming in on the molecular structure of feathers to understand how different colours are formed.
There are two basic mechanisms for creating colour in nature: via structure or chemical pigments. Blues are often the result of microscopic structures that bounce and scatter light, leaving only the shorter wavelengths—shades of blue—visible. Reds, yellows and oranges usually come from chemical pigments: molecules such as melanin, which makes human skin darker, or carotenoids, which give carrots and autumn leaves their orange, red and yellow hues. These chemicals absorb some wavelengths of light but reflect other wavelengths: the russet, gold or brown that other animals see.
Withers plucks a tiny feather, no bigger than a fingernail clipping, and places it in a plastic sample bag. Later, she will dip it in resin, slice a thin section and place it under a high-powered scanning electron microscope to produce an extreme close-up image, showing her details that are as small as two microns—about one-50th of the width of a human hair.
The image will reveal that inside the males’ feathers there is a cobweb of keratin, the molecule that makes up your fingernails. Light bounces through this maze of keratin and empty space, emitting only blue wavelengths. Overlaying the keratin are yellow carotenoids—resulting in the lime-and-khaki of males. Withers also learns that females lack the keratin structure, but carotenoids in combination with melanin produce the darker spots in their speckling.
“Carotenoids are fascinating because birds can’t create their own,” says Withers. In other words, birds have to eat them. “This means the strength of yellow or green pigmentation could be an indicator of an individual’s health and robustness. This is why I think green is more interesting than we think it might be: there’s a lot of potential for signalling and communication.”
Perhaps, instead of camouflage, the rifleman’s khaki is ladies’ choice: greener males might be more attractive to the females. For some other bird species, ultraviolet plumage is important for attracting a mate. When scientists covered up the ultraviolet-active plumage patches on budgies with sunscreen (which blocks UV light), they found that females preferred the male whose UV colouration they could still see.
Back in Hawke’s Bay, Cain and her team monitor the rifleman population like an FBI surveillance squad. The birds are fitted with ankle bracelets—leg bands that identify each individual—and their every move is recorded with shotgun mics, cameras and watchful eyes through binoculars.
“There’s nothing that quite prepares you for working with tītipounamu,” says Cain. “They have a high-pitched call that’s hard to hear, and they’re hard to spot and follow, particularly if it’s windy. Nests are really difficult to find—you have no idea where to look. It could be 20 metres up a giant mataī or it could be on the ground under some leaves.”
That’s one thing that can be guaranteed when it comes to riflemen: they will continue to challenge accepted scientific wisdom.
In birds, the ability to detect ultraviolet comes down to a single gene mutation. In a study that surveyed the genetic sequences of a variety of different birds, the rifleman had a type of mutation not seen before. We still don’t know what this means for how they perceive colour. “Riflemen are strange,” says Withers. “Sometimes you think you’ve really nailed them, and then it’s like, ‘No, that’s not what’s happening. They’re always different from what you expect’.”
Cain laughs. “Yeah, if you go in expecting them to be a normal songbird, you’re going to be really disappointed.