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.
This is costly to a host pair because their own young lose out. With starvation already widespread among coot broods98 percent suffer at least one deaththe 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 ducksanother 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 aboutan 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.
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 haveand which ones are theirsand 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.
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