From the vivid feathers of rainbow lorikeets to the bright blue bottoms of mandrills, the tropics are certainly a flamboyant feast for the eyes. Writing in the 19th century, Charles Darwin was struck by this, remarking on the “rich variety of colours”.
This is borne out by modern research. A team led by the University of Sheffield studied more than 4,500 different songbird species from around the world, preserved in London’s Natural History Museum, and found that tropical songbirds are more colourful than their non-tropical counterparts.
The feathers of those species living close to the equator display around 20–30 per cent more colours than those of birds living at higher, non-tropical latitudes. This is true for male and female birds, even though they can sometimes look very different from each other.
Why are tropical birds more colourful?
To find out why, the scientists then looked for links between the birds’ colouration and their ecology. They found that colour diversity was highest in birds from dense, closed habitats such as rainforests, and in those that eat nectar from flowers and fruits. This suggests two reasons to explain the vibrancy of tropical birds.
The first is that, in the dense, green tropical forests, bright colours help the birds stand out. Vibrant hues and distinct vocalisations can help birds to locate and communicate with each other, especially during mating rituals or territorial disputes.
The second is that birds are acquiring pigments from the foods that they eat, which then affect their colouring. An example of this is the stunning scarlet ibis. The wading bird, from South America, feeds on shrimp and other shellfish, which produce a red pigment called astaxanthin. This, in turn, gives the bird its unmistakable vermilion hue. The same ecological features are thought to contribute to the colourfulness of other, non-avian tropical species. But that’s not all.
Mandrills have blue bums, not because of any pigment but because of the way the collagen fibres are arranged in their skin. This scatters light in such a way that their behinds look blue – in a similar way to how we see the sky as blue.
Higher ranking males (with more testosterone) have brighter rumps, faces and genitalia than lower ranking individuals. Females interpret this as a sign of ‘fitness’. So, in this case, the flashy feature’s evolution has been driven by sexual selection.
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Top image: bananaquit birds in Curaçao. Credit: Getty
