According to research by Chris Elphick, 95% of female Saltmarsh Sparrows mate with more than one male to produce each clutch of eggs. The average nest had eggs fertilized by 2.5 different fathers. One-third of nests had a different father for every chick. This is a key part of their struggle to produce young that survive and fledge.
The saltmarsh sparrow (Ammodramus caudacutus) is a small, stocky bird that lives along the US Atlantic coast.The Saltmarsh Sparrow has the highest documented rate of extra-pair mating. However, other birds may have higher rates that simply have not been documented yet. The Greater Vasa Parrot (Coracopsis vasa) and the Superb Fairywren (Malurus cyaneus) have comparable rates.
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.
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.