There has always been the presumption or assumption that tropical birds are generally more colorful than those further from the tropics. The closer the birds live to the equator, the more colorful they are, we have always thought. Alexander von Humboldt, Charles Darwin, and Alfred Russell Wallace, well-known naturalists of the 19th century, all noted the “rich variety” and “mixtures of colors” that they found in their travels. Is this true and why would that be?
Well, in general, tropical forests are denser and darker than other terrestrial habitats, making it more difficult for birds to find each other, whether it be a potential mate or competitor; colors make identification easier. Also birds in areas of high bird species diversity like the tropics tend to be brighter to help them stand out.
Tropical forests also have an abundance of food and a benign climate, allowing the birds to put more energy into showy characteristics like plumes or crests, plumage color, and/or behaviors. Or perhaps the pigments in the variety of fruits available in the tropics just makes it easier to develop colorful feathers.
There has been little research on the subject, although a 1978 paper dismissed the idea. But a recent paper entitled Latitudinal Gradients in Avian Colorfulness, whose findings were published in the journal Nature Ecology & Evolution, substantiates the idea. According to one of the investigators, “We tested whether a latitudinal gradient in species’ colorfulness exists for the global radiation of passerine birds (order Passeriformes), the largest avian order, comprising 60% of the 10,000 bird species.” The researchers analyzed over 140,000 visible and ultraviolet light photographs of museum specimens for 4,527 songbird species.
The researchers classified the color of the plumage on 1500 pixels of the photographs. “This work reveals the broad pattern that bird species tend to be 30% more colorful towards the equator ….,” according to lead researcher Dr. Christopher Cooney. Why this is is unclear but one suggestion is that the variety of plumage colors are related to the variety of food sources.
Darwin, like other naturalists of his time, reported on his findings, often requiring the description of colors. During the voyage of the Beagle he depended upon “Werner’s Nomenclature of Colours,” published in 1814 by the Scottish artist Patrick Syme and containing the descriptions and samples of 110 different colors. A hundred years later in 1912 ornithologist Robert Ridgway published Color Standards and Color Nomenclature, describing 1115 colors for birds.
How many colors do birds have? Are there really 1115 colors? Birds have better color vision than us so maybe there are 2500 colors. Or perhaps birds ignore certain color ranges; to them, maybe royal blue, baby blue, cerulean blue, navy blue, cobalt blue, sky blue, etc. are just blue to them?
Benjamin Moore lists 3500 colors of paint. Really. When I look at all the paint samples on little cards in a rack at a hardware store, it’s stupefying. To me there’s dark blue. light blue and in between. But I’m not a bird.
Being an artist, I often marvel at the pairings of colors between certain species and their preferred food for the day. The blue-eyed Streaked Saltator gorges on small flowers of almost the same delicate blue (on a vine) — but this year I watched a Streaked Saltator gobble bright-red flowers on another vine. That helped me dismiss the hypothesis about blue flowers and equally-blue eyes. And then I wonder how do birds like the Great Antshrikes or Whooping Motmots have such startling red eyes. My favorite bird-fruit pairing is when the handsome Orange-fronted Barbets are raiding the Starfruit/Carambola tree.
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