Hummingbirds are among the brightest birds in the world, and now scientists know why.
A new study in the journal Evolution reveals that while their feathers have the same basic makeup as those of other birds, the shape of their pigment-containing structures enables them to reflect a rainbow of light.
An international team led by Chad Eliason from the Field Museum, US, used transmission electron microscopes to examine the feathers of 35 species of hummingbirds and compare them with those of brightly-coloured birds, such as green-headed mallard ducks.
They found that secret lies in the melanosomes – the pigment-producing organelles that are part of the cells that make up the barbules that project from the barbs of bird feathers.
The shape and arrangement of melanosomes can influence the way light bounces off them, producing iridescent colours that the researchers call structural colours because they depend on structural dimensions.
Hummingbird melanosomes are special, they discovered. They are flat and contain lots of tiny air bubbles, which creates a more complex set of surfaces. When light glints off those surfaces, it bounces off in a way that produces iridescence.
Eliason and colleagues also found that the traits that make hummingbird feathers different – the melanosome shape and the thickness of the feather lining – evolved separately, allowing hummingbirds to mix and match a wider variety of traits.