This story is wrong, at least for eider ducks. Ralph Tiedemann from the University of Potsdam has shown that among these birds, brood parasites are most likely to target their own relatives, especially older ones who lay smaller clutches of eggs themselves. They aren’t putting their babies on the doorsteps of random strangers; they’re offering them to Grandma. The ‘victim’ isn’t really a victim; she’s the family babysitter.
When the time comes to breed, eider ducks tend to return to the place where they hatched. This trait brings several generations of eiders to the same place, and thousands of birds can gather in coastal islands.
Tiedemann worked with two such eider colonies: one that breeds in the Southern Baltic Sea, and another that nests on an airport runway in Northern Iceland. He took blood samples from 140 females and their 506 ducklings, and analysed seven parts of their genome to work out how related they were to one another.
He found that around a third of the nests contained at least one “parasitic” duckling, and in these nests, around 40% of the chicks had come from a different female. But the eiders are more likely to dump their eggs in the nest of a close relative than those of unrelated neighbours, something that Malte Andersson had found in a group of Canadian eiders in 2007. These eiders aren’t raising a stranger’s chicks – they’re probably raising their nieces, granddaughters or great-granddaughters.
Tiedemann also found that the eiders are more likely to raise parasitic ducklings, the older they get. Around one in six nests belonging to young females (three to six years old) contained parasitic chicks. That proportion doubled for females older than seven. Tiedemann even found three nests where every single duckling was an alien one – all of these nests belonged to very old eiders (15 years or older).
Older females lay fewer eggs than younger ones, but Tiedemann thinks that they can rear more chicks than they actually produce. Their best strategy for contributing to the next generation is help rear the chicks of relatives who bear many of the same genes. Meanwhile, younger females, who produce more eggs than they can care for, can entrust the surplus to their relatives.
It’s not clear whether the same is true for other birds. Brood parasitism is remarkably common among birds in general, and ducks in particular. Scientists have documented the practice in 234 species of birds, a third of which are ducks. The eiders might be a special case, since they breed in big colonies that span the generations. But for them at least, brood parasitism is clearly not a simple case of grifters and marks. It’s more a case of families, cooperating across the generations.