By Julianna Kettlewell
BBC News Online science staff
A type of parasitic bird, which is reared by unrelated host "parents", is happy to share its nest with the host's babies, Science magazine reports.
Traditional theory predicts that parasitic birds should push host chicks from the nest
Traditional theory predicts that parasitic birds should push host chicks from the nest, to avoid sharing food.
But a transatlantic team have shown that cowbirds actually grow faster if the host chicks remain in the nest.
They speculate that host parents bring much more food to a full nest, which the cowbird can get a lion-share of.
Brown-headed cowbirds live in the grasslands of North America. They carve a living by following herds of bison or cattle, and feeding off the insects that the beasts attract.
This occupation requires a rather nomadic life-style: the cowbirds are obliged to go where the herds go. This makes bringing up young problematic.
Nest-building and chick-rearing fixes you to the spot for a while, which the cowbirds can little afford.
They get over this obstacle by becoming "brood parasites". In other words, they recruit different bird species to unwittingly raise their young.
Very simply, they lay their eggs in the nest of an unsuspecting song-bird, like the eastern phoebe, and leave.
This is a well known strategy among birds - several species do it, including cuckoos.
But the cowbird sparked interest because of its seemly benevolent attitude towards its "foster siblings".
On the face of it, it makes sense for a parasitic chick to eject the host's real babies from the nest. After all, they are no relation of the impostor, and they consume valuable food.
Cuckoos certainly abide by this rule. As soon as a cuckoo chick hatches, it quickly dispatches of its fellow nest-mates, meaning it never has to share.
But other parasitic birds, like the cowbird, are apparently rather kinder. Although bigger than the host chicks, they generally avoid pushing them out.
Of course, in reality, kindness has nothing to do with it. Some scientists believed that parasitic birds who tolerated the host young were just slow to evolve the "aggressive" strategy.
"The traditional view was that there was no adaptive benefit to be gained by sharing a nest," co-author Rebecca Kilner of Cambridge University, UK, told BBC News Online.
"So the idea was that the brood parasites that did this were just suffering from evolutionary lag - in other words, it is a trick they have yet to develop."
But when Dr Kilner and her colleagues studied the brown-headed cowbird they discovered the parasites probably have a very selfish reason for putting up with their nest-mates.
Their results show that cowbirds who share their nest with two host chicks actually thrive: they grow much faster than cowbird chicks reared alone.
The cowbirds lay their eggs in the nest of an unsuspecting song-bird and leave
"We found that cowbirds grew up best when they had about two host young with them," said Dr Kilner.
The team speculate this is because the host parents work disportionatly hard to feed a full nest.
The "foster" parents will rush back and forth bringing far more crushed flies to the nest than if they have a single chick to care for.
And, because the cowbird chick is rather more robust, it gets more than its share of the food.
"We measured how often the parents came to the nest with food and found they came far more frequently when there were three chicks in the nest than if the cowbird was by itself," explained Dr Kilner.
"But the cowbird gained disproportionately from the higher provisioning because it could overpower the host young, and take more than its fair-share of food."
This research is raising a question in many biologists minds: If it is such a good idea for parasitic birds to share a nest, why don't they all do it?
Dr Kilner thinks it might have something to do with the size of the parasitic bird in relation to the size of the host. If the parasitic bird is relatively big - like cuckoos are - then they simply cannot afford to share, she believes.
"One possibility is if the brood parasite is quite large in relation to its host bird, then there might not be enough food to go around for both the parasite and the host chicks," said Dr Kilner.
"So the best thing for the parasite young to do is kill the host young so that it gets all the food.
"But it is a question we would all like answered."
The research was carried out by Rebecca Kilner and Joah Madden from Cambridge University and Mark Hauber from the University of California, Berkeley, US.