Mates or survival: Which explains a bird’s color?

The famous scientist Charles Darwin spent a lot of time watching birds. When he saw male birds with bright colors, he thought that their colors were due to sexual selection — adaptations that help an individual get a mate. Males that impress a female with their plumage, for instance, are more likely to mate and have offspring. But Darwin’s fellow scientist Alfred Russel Wallace watched drab female birds. He credited their more boring colors to natural selection — adaptations to help a species survive. Drab colors might help females hide from predators, upping their survival rate.

So who was right: Darwin or Wallace?

In an attempt to answer this, a new study analyzed thousands of bird specimens. And its authors now conclude that both Darwin and Wallace were right.

To conduct this new research, scientists didn’t hunt down all of those birds in the wild. Instead, they spied them in museums.

That’s right. Museums.

These are far more than places where you can get up close to dinosaur bones (though that’s certainly a lot of fun). Museums often have huge collections of animal and plant samples that tourists never see. Scientists use museums to study organisms that would be difficult to catch in the wild, that may have gone extinct or that lived in vastly different periods of time. 

Their research collections also can save scientists a lot of time when they are trying to study many species.

As an ornithologist (Or-nih-THOL-uh-jizt), Peter Dunn studies birds. “I’ve always been interested in why birds are bright and colorful,” explains this scientist at the University of Wisconsin, Milwaukee.

He’s been especially interested in learning what drives bird colors: natural selection (getting more food, say, or avoiding predators) or sexual selection (attracting a great mate). In peacocks, for example, only males get that fantastic feathered fan. Similarly, among fairy wrens, the guyshave brightly colored heads and necks, while the gals are brown and drab.

At first, Dunn’s group just eyeballed differences among bird species. “We were going to the museums, looking at the bird colors with our eyes and judging whether the male was different from the female,” he recalls. But then they decided to make their analyses more precise.

So they turned to a spectrophotometer (SPEK-troh-foh-TOM-eh-ter). This instrument measures the light reflecting off of a material, such as a feather. With this tool, Dunn explains, “we can say for certain that it’s a specific shade of blue or green without relying on human vision.”

The scientists scanned birds in five museums in the United States and Australia. They analyzed the color of three males and three females from 977 different species. Clearly, Dunn says, museums were the best way to quickly study so many birds. “It would take forever to catch them and use a spectrophotometer out in the wild.”

The scientists compared a feather’s hues to the different types of challenges that its species might face. Does it have to find a mate quickly or risk losing the chance to mate this year? Do females of its species nest on the ground, where predators might easily get them? Might bright colors scare off prey as birds fly overhead? Each of these conditions can affect how a bird behaves — and could be linked to the hue of its feathers.

When male and female birds are the same color, natural selection likely caused them to evolve plumage of same hue, the new study concludes. “Natural selection determines the color in general,” Dunn says. “For example, many seabirds [such as seagulls] are white. It’s so they can forage for fish.” When those birds hunt, fish can’t see them easily when they look up. The birds’ white bellies blend in against the sky. So here, both sexes are the same color.

But when males are brightly colored and females dull, sexual selection is more likely to be the reason, Dunn’s group argues. If a male bird is an attractive color, then getting the gals is probably a bigger challenge for him than finding a good meal. 

The paper “helps us to understand where diversity comes from, and why we see what we see in nature,” says Jordan Price. He’s an ornithologist at St. Mary’s College of Maryland in St. Mary’s City. He notes that this is not the first study to look at how bird color evolved. But it is one of the largest. And it carefully studied males and females to probe how much of a role natural and sexual selection play in vibrant plumage. 

The new data also point to why studying female bird color is important. “Males [often] are the conspicuous ones,” Price says. Females might be harder to spot, but when it comes to understanding species survival, their traits deserve equal study.

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Power Words

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adaptation  A process by which an organism or species becomes better suited to its environment. When a community of organisms does this over time, scientists refer to the change as evolution.

evolution  A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

infrared light  A type of electromagnetic radiation invisible to the human eye. The name incorporates a Latin term and means “below red.” Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X rays, radio waves and microwaves. It tends to record a heat signature of an object or environment.

ornithology  The scientific study of birds. Experts who work in this field are known as ornithologists.

natural selection   This is guiding concept underlying evolution, or natural adaptation. It holds that natural mutations within a population of organisms will create some new forms that are better adapted to their environment. That adaptation makes them more likely to survive and reproduce. Over time, these survivors may come to dominate the original population. If their adaptive changes are significant enough, those survivors may also constitute a new species.

sexual selection  A subtype of natural selection, in which some animals have adaptations that are more attractive to mates, and make them more likely to reproduce.

spectrometer  An instrument used by chemists to measure and report the wavelengths of light that it observes. The collection of data using this instrument, a process is known as spectrometry, can help identify the elements or molecules present in an unknown sample.

spectrophotometer   A machine that measures light reflecting off a material, and is limited to visible light, near-infrared, and near-ultraviolet. It can be used to measure the relative color of solids or liquids. It can also be used to measure the color of a bird’s feathers or an animal’s skin.

ultraviolet  A portion of the light spectrum that is close to violet but invisible to the human eye.