Hummingbirds owe their famous iridescent plumage to ‘pancake-like’ cells in their feathers, according to new research.
An international team of scientists carried out the largest optical study of its kind to find out why the birds, which are native to the Americas, shine so brightly.
Hummingbird feathers display intense iridescence – they appear to gradually change colour as they are viewed from different angles, like light off a soap bubble.
No other bird seems to have the iridescence of a hummingbird, but scientists weren’t sure why.
After examining 35 different species of hummingbirds under microscopes, they discovered it was due to the shape and arrangement of melanosomes – tiny structures within cells that synthesise light-absorbing pigment.
The pancake-like flatness of these melanosomes influences the way light bounces off them, giving a greater array of colours.
A close-up of a Ruby-throated hummingbird feather with a red iridescence, which is highly directional and appears as a dull black from several angles
‘We call these iridescent colours ‘structural colours’ because they depend on the structural dimensions,’ said co-author Professor Matthew Shawkey, a biologist at the University of Ghent, Belgium.
The international team examined the feathers of 35 species of hummingbirds with transmission electron microscopes.
Then they compared them with those of other brightly-coloured birds, like green-headed mallard ducks, to look for differences in their make-up.
All birds’ feathers are made of keratin, the same material as human hair and nails, and are structured like tiny trees, with parts resembling a trunk, branches, and leaves.
The ‘leaves’, called feather barbules, are made up of cells that contain pigment-producing organelles, or cells, called melanosomes, which produce the dark melanin pigment that colours people’s hair and skin.
The shape and arrangement of melanosomes can influence the way light bounces off them, producing bright colours.
‘A good analogy would be like a soap bubble. If you just look at a little bit of soap, it’s going to be colourless,’ said Professor Shawkey.
‘But if you structure it the right way, if you spread it out really thin to form the shell of a bubble, you get those shimmering rainbow colours around the edges.
Costa’s Hummingbird with a stunning iridescent purple throat feathers catching the light
‘It works the same way with melanosomes. With the right structure, you can turn something colourless into something really colourful.’
While ducks have log-shaped melanosomes without any air inside, hummingbirds’ melanosomes are pancake-shaped and contain lots of tiny air bubbles.
The flattened shape and air bubbles of hummingbird melanosomes create a more complex set of surfaces.
When light glints off those surfaces, it bounces off in a way that produces iridescence – the effect of luminous colours appearing to change when seen from different angles.
‘In mammals, the melanin isn’t organised in any fancy way inside of the hairs, but in birds, you get these layers of melanosomes, and when light bounces off the different layers, we see bright colours,’ said Dr Chad Eliason at the Field Museum in Chicago and the study’s first author.
Broad-billed hummingbird with bright turquoise or metallic green feathers on their upper parts and breast
The study also found that the different traits that make hummingbird feathers special – like melanosome shape and the thickness of the feather lining – are traits that evolved separately.
There are more than 350 species of hummingbirds, which live exclusively in the Western Hemisphere, from Alaska to the tip of South America.
‘Not all hummingbird colours are shiny and structural – some species have drab plumage, and in many species, the females are less colourful than the males,’ said co-author Rafael Maia, a biologist and data scientist.
The study, which has been published in Evolution, opens the door to a greater understanding of why hummingbirds develop the specific colours that they do.
