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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