Bright Beaks Signal Health to Female Birds, Study Says

John Pickrell
for National Geographic
April 3, 2003
Flamboyant male ornaments such as peacocks' tails, crabs' claws, roosters' combs, and stags' antlers have long been thought to advertise the quality of a potential partner to discerning females in the market for a mate.

Now, researchers in France and the United Kingdom report the first experimental evidence that brightly colored beaks, in two different bird species, are directly linked to the quality of the immune system.

"It's been known for a long time that females of many species choose to mate with the flashiest males," said Jonathan Blount evolutionary biologist at the University of Glasgow in Scotland. "Quite what they stand to gain from mating with these show-offs has been puzzling ecologists since the time of Charles Darwin," he said.


Now, Blount and his co-authors working with zebra finches in the U.K., and an independent research team working with blackbirds in France, think they may have found proof that bright red or orange beaks attract females because they advertise that their male owners are in top-notch physical condition. The findings are detailed in two studies published in the April 4 edition of the journal Science.

Previous studies had noted that food-derived plant chemicals, called carotenoids, are necessary to produce many of the bright red to yellow colors that animals display, commented freelance evolutionary biologist George A. Lozano, formerly of McGill University in Montreal, and now based in Ottawa. Examples include the red markings of salmon, stickleback, and guppies, in addition to many birds.

These chemicals are also behind many similar plant colors, such as that of the bright red leaves of some species, said Lozano. Animals are unable to synthesize carotenoids, so access to them is limited by the amount they can harvest from their diet.

Researchers have also found evidence that these same chemicals are important for a healthy immune system in animals, mopping up damaging metabolic by-products, and boosting the body's defenses, for example.

To test the link between carotenoid-dependent ornament flashiness and immune function, Blount's team gave carotenoid-supplemented water to ten zebra finches (Taeniopygia guttata) over an 8-week period. Over the same time frame, another group of finches, made up of a single brother of each of the birds in the carotenoid-supplemented group, was given untreated water.

The scientists found that the finch brothers provided with dietary supplements, developed significantly redder beaks by the fourth week of the study.

Furthermore, in additional tests, the researchers found, not only that nine out of ten females preferred to perch near carotenoid-treated, bright-beaked males, but also that these males exhibited more potent immune responses than their un-supplemented siblings.

Tale of Two Cities

At the same time, and unknown to Blount's team, an independent team of researchers, led by biologist Bruno Faivre at the University of Bourgogne in Dijon, France, was testing the link between bright beaks and immune function in male blackbirds (Turdus merula).

The French researchers approached the problem from a different angle, however, stimulating the immune system of healthy blackbirds in the laboratory. Unlike dowdy, brown females of the species, male blackbirds possess bright yellow-to-orange beaks and shiny black plumage.

As carotenoids can be mobilized by the body, as and when required, the researchers predicted that otherwise healthy blackbirds should lose some of the bright orange vibrancy of beak color when the chemicals need to be reallocated for defensive purposes.

The scientists found that within three weeks, bill color significantly decreased in those males experimentally immunized to stimulate an immune response. "We didn't think the answer could be so quick", said biologist Frank Cézilly co-author at Borgogne. "In blackbirds, dynamic reallocations of carotenoids from the beak to the immune system appear to convey a continual update on male health," he said.

In the wild, "Only those [male birds] with the fewest parasites and least disease, are likely to be able to allocate enough carotenoids to produce the best…displays, " said Blount. The benefits for a female selecting a healthy mate are likely to include "a reduced risk of picking up parasites and diseases and better chance of having a mate that will go the distance in rearing offspring," said Blount.

Females could possibly additionally benefit by begetting offspring that share the good genes responsible for their father's health, though this remains to be tested.

Though many of the pieces of the puzzle had been examined in prior work, "Everything has been put together very neatly" in these studies, said Lozano. "Both support the idea that carotenoids have a role in sexual selection [by females]," he said.

Intriguingly, said Blount, some work has suggested that carotenoids and related antioxidant vitamins could be responsible for delaying some of the physical effects of aging in people, such as wrinkly skin. It would be interesting to investigate whether people with higher levels of such antioxidants retain a more youthful appearance later in life, and are therefore more attractive mates, he said.

Recent Bird Stories by National Geographic News:
Coot Birds Can Count, Study Says
Falconry Used to Secure North American Airports
Cuckoos, Wrens in Escalating Evolutionary Arms Race
Deer Behind Britain's Great Bird Decline?
"Mysterious Plague" Spurs India Vulture Die-Off
Gamblers Fuel Trade in "Lucky" Vulture Heads in Africa
Ospreys Flock to Cuba, With Conservationists Close By
Sixth Great Backyard Bird Count Begins in U.S.
Bird Story: Black-Capped Vireo—Hope for Survival?
Four-Winged Dinosaurs Found in China, Experts Announce
Aggressive Seagulls Menacing Urban Britain
Satellites Help Reveal Secrets of Epic Goose Migration
Birds May Hold Clues to Role of Time in Teamwork
Mysterious Kenya Flamingo Die-Offs Tied to Toxins, Study Says
Quarter of U.S. Birds in Decline, Says Audubon
Farmers, Conservationists Seek Return of Barn Owls
Seasons of a Birder's Life
Do Some Birds Cheat to Avoid Inbreeding?
Water-Diversion Plan Threatens California's Salton Sea

National Geographic Bird Resources:
Bald Eagles: Come Back From the Brink
Experience the Sights and Sounds of Eagles
Snowy Owls—Muscle & Magic
Attwater's Prairie-Chickens—Down to a Handful

Recent "Birder's Journal" Stories from Robert Winkler:
Giving Thanks for Wild Turkey Sightings
Birder's Journal: Ghost Town's Curse Haunts New England Forest
Birder's Journal: Looking at a Handy New Guide
Birder's Journal: Learning to Let Birds Come to You
Birder's Journal: A Morning With Migrants
Birder's Journal: This Warbler Is a Master of Deception
Birder's Journal: Seduced by Dueling Thrushes
Birders Journal: Attack of the Flying Goshawk Bird-Watching Sites:
Boston Area
Chicago Area
Florida Keys Area
Maine's Acadia National Park
Mount Rainier
New Orleans Area
New York City Area
North Carolina's Outer Banks
Philadelphia Area
Portland Area
Rocky Mountain National Park
Salt Lake City Area
San Francisco Area
Santa Fe Area
South Dakota's Black Hills
Washington's Olympic National Park
Wyoming's Grand Teton National Park
Yellowstone National Park
Yosemite National Park

From the National Geographic Store:
Guide to North American Birds
Portable Birdsong Identifier
Birder's Journal
Songbirds Puzzle

Additional Information from Related Web Sites:
American Bird Center
American Bird Conservancy
Fish and Wildlife Service Bird Web Site
National Audubon Society
Environmental Protection Agency: Bird Conservation

© 1996-2008 National Geographic Society. All rights reserved.