By Jody Bourton
Earth News reporter
The saltmarsh sparrow has 'wild' mating habits
A bird living on the coast of the US is the world's most promiscuous bird, say scientists.
The saltmarsh sparrow, a bird that lives in the marshes of Connecticut, was found to have extreme levels of multiple mating.
The researchers found that 95% of females mated with more than one male during each nesting period.
This unusual behaviour could be a survival mechanism due to coastal flooding, researchers say.
The researchers, who are based in the US, publish their results in the journal The Auk.
Using DNA analyses and studying the birds mating behaviour in the marsh habitat, the scientists revealed the highly promiscuous activities of the bird.
"We found that nearly every clutch of eggs was the product of more than one father, and that within broods it was extremely common for any two siblings to have different fathers," says Professor Chris Elphick from the University of Connecticut.
Professor Elphick undertook his research along with Professor Christopher Hill from Coastal Carolina University in South Carolina, US and Carina Gjerdrum of the Canadian Wildlife Service.
The scientists found that at least 95% of females mate with more than one male for a single clutch of eggs.
A clutch is defined as a set of eggs laid together in the nest at one time.
One in three nests had a different father for every chick, and the average brood of chicks had more than 2.5 fathers.
"The chance that any two chicks in the same nest have the same father is only 23%," says Professor Elphick.
"We were not surprised to find some level of promiscuity," he says. "But we were quite stunned at just how extreme the rate was."
'Eggs in one basket'
The saltmarsh sparrow (Ammodramus caudacutus) is a small, stocky bird that lives along the US Atlantic coast.
Some of their behaviour is unusual for songbirds; males and females do not bond together to form pairs, and the males play no role in caring for chicks.
The sparrows nest amongst the saltmarshes, and are vulnerable to frequent high tides, which can cause a high level of nest loss.
Very high tides occur every four weeks - the same length of time it takes for the sparrow to raise a family.
Who's your father?
Professor Elphick suggests that the mating patterns are are a response to this risky environment.
"If they lose their young to flooding, they have to re-nest almost immediately if the new set of young is to survive," he says.
This means that female birds do not have time to look for and invest in the best male partner.
The lack of time increases the likelihood of choosing a poor quality mate. To overcome this, it seems that females mate with several males.
"The females don't want to put all their eggs in one basket so to speak," says Professor Elphick.
"We think that it is the most promiscuous bird species studied to date, although there are a couple of other possible contenders," says Professor Elphick.
The greater vasa parrot (Coracopsis vasa) of Madagascar, and the superb fairywren (Malurus cyaneus) of Australia have comparable rates of promiscuity.
"Both of these species also have multiple paternity in most nests, but it is unclear whether they have so many fathers per nest."
The differences between studies that have been carried out into each of the bird species mean it is impossible to make a direct comparison.
But their extreme promiscuity is not the most interesting thing about these birds, says Professor Elphick.
"What is most interesting about these three species is that all have totally different social systems," he explains.
"Unrelated species have all converged on high levels of promiscuity through very different sets of behaviours."
"It's the multidimensional complexity of all those species - the many ways in which they differ from one another that makes the natural world so amazing."