National Geographic News: NATIONALGEOGRAPHIC.COM/NEWS
 

 

Coot Birds Can Count, Study Says

James Owen in England
for National Geographic News
April 2, 2003
 
To most people, coots are noisy, quarrelsome water birds that do a lot of splashing about. But it turns out they are also closet cuckoos. Not only that, they can count.

The discovery was made by Bruce Lyon, a biologist at the University of California, Santa Cruz. His study of an American coot colony in British Columbia, Canada, is the first to show that birds can keep a reckoning of the eggs they lay. It also highlights an extremely rare example of counting by a wild animal.

Supported in part by the National Geographic Society's Committee for Research and Exploration, the research is published this week in the science journal Nature.

While cuckoos are interspecific brood parasites (they lay eggs in nests belonging to other bird species), American coots are conspecific (they target their own species).

There are various reasons why they do this.

Female coots that lose out in the battle for breeding territory will try and offload eggs onto successful nesters. Also, the last few eggs laid in a clutch have a reduced chance of survival. Females that lay them elsewhere should improve their overall breeding success.


In addition, coots have more opportunities for conspecific parasitism than many other birds, according to wildlife biologist John Eadie, from the University of California (Davis) department of wildlife, fish, and conservation biology.

"Coots have large clutch sizes so the number of days during which a host can be found and parasitized [before incubation starts] is greater," said Eadie. "American coots also nest at quite high densities so there are more potential host nests available."

Lyon's four-year study found that conspecific brood parasitism was commonplace among American coots. Over 40 per cent of pairs were targeted, with an average 3.1 eggs left in each nest.

Chick Starvation

This is costly to a host pair because their own young lose out. With starvation already widespread among coot broods—98 percent suffer at least one death—the burden of having extra hungry mouths to feed means even more fatalities. If a parasitic chick survives, one of the host's young is likely to have died.

American coots have responded to this threat by learning to recognize eggs that aren't theirs. Lyon found that 43 percent of host pairs rejected at least one parasitic egg before it hatched.

This ability is unusual among birds. Apart from coots and related rails, only ostriches and weaverbirds can detect parasitic eggs left by their own species.

During a previous study in Argentina, Lyon found that red-gartered and red-fronted coots were eagle-eyed detectors of eggs belonging to black-headed ducks—another bird with a cuckoo-like attitude towards parenting.

American coots are similarly adept at spotting differences between the background color of their own and other eggs. The more a parasitic egg differs from that of the host, the greater the chances of rejection. These eggs are then buried within the nest's structure.

Host pairs also operate a more subtle form of discrimination. Instead of burying foreign coot eggs, they are banished to the nest's periphery. This puts them in a less favorable incubating position, so giving the host chicks an important head start in life.

Lyon suggests this fate awaits eggs the coots aren't too sure about—an insurance policy in case they make a mistake. If they do, then at least their own offspring still have a chance of surviving.

However, the American coot's most surprising defense against sneaky egg-layers is its ability to count.

Egg Counting

Females usually stop laying when their overall clutch reaches a certain size, but those that discriminate against parasitic eggs keep on going. This suggests they can tell how many eggs they have laid themselves.

"I found that birds that are unable to distinguish parasitic eggs lay one fewer egg for each parasitic egg received," Lyon explained. "In contrast, birds that ultimately reject parasitic eggs do not reduce their clutch sizes. It's the comparison of these two different responses that really indicates counting."

Scientists say it's an important discovery. Although some animals have been observed counting under laboratory conditions, examples from the wild are almost non-existent.

"Many animals apparently have a brain-wiring that in the right circumstances can support competent counting without verbal symbolic representation of numbers," said Malte Andersson, from the University of Gothenburg, Sweden. "Lyon's findings provide a fascinating example of how this capacity is put to good use in the wild."

"The study raises some very interesting questions about just how animals are able to assess the number of young they have—and which ones are theirs—and adjust their parental care accordingly," Eadie said.

Lyon believes visual egg-counting could be a skill common to many birds.

An ability to add up and regulate the number of eggs in a clutch would maximize breeding success by preventing overproduction, so cutting deaths from starvation. Such monitoring would also reduce the incidence of undersized broods.

This same 'family planning' strategy may also be used by mammals.

Meanwhile, the search continues for other birdbrains with a gift for numbers. If the American coot is anything to go by, there are plenty more to be found.

Recent Bird Stories by National Geographic News:
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

Nationalgeographic.com 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
Utah
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.