Are you intimidated by this bird? Would you be if it destroyed all your unborn children?
Some species of cuckoo and other birds are “brood parasites.” This means that instead of raising their own young, they sneak their eggs into other birds’ nests and let the adoptive parents do all the work. If you watch these parents taking care of hulking nestlings that are clearly a different species from their own young (and sometimes bully their adoptive siblings out of the nest), it’s hard not to see the parents as suckers. But there’s evidence for another possibility: that adoptive parents accept baby cuckoos into their nests because they fear violent retaliation if they don’t.
The “mafia hypothesis” is based on scientists’ observations that great spotted cuckoos (above) and brown-headed cowbirds (another brood parasite) can retaliate against birds that oust their eggs. If a host parent tosses a cuckoo or cowbird egg of out its nest, the cuckoo or cowbird parent may return and destroy every other egg that bird has laid. “Mafia” might imply more organization than is really involved—but the idea is that host parents adopt new eggs because they know there’s a threat if they don’t.
To find out whether the controversial mafia hypothesis might be true, Maria Abou Chakra and her colleagues in the Evolutionary Theory Group at Germany’s Max Planck Institute ran a series of mathematical simulations. It all comes down to game theory—if murdering baby birds were a game.
In the simple version of their model, researchers assumed that each cuckoo (or other brood parasite) can lay only one egg per nest. They assumed two types of cuckoos: “mafia” birds, which after being rejected will destroy a host’s entire nest, then return to lay a new egg; and non-mafia birds, which just move on to a new host if their egg is kicked out. And the researchers assumed three types of host birds: accepters, who always take the cuckoo egg; conditional accepters, who only accept an egg if they’ve already experience retaliation; and rejecters.
Every action has a cost. For a host bird, raising a cuckoo baby along with its own young is expensive. But kicking out the cuckoo egg risks getting its whole nest killed, then having to lay all new eggs. For the cuckoo, losing an egg is costly—but so is spending time and energy to monitor the host nest and retaliate if necessary.
In this model, the authors saw a cyclical pattern emerge. When most cuckoos are non-mafia, it makes sense for hosts to be conditional accepters: they’re probably safe kicking out the first cuckoo egg, but if the cuckoo retaliates they’ll take the hint. This creates a fitness advantage for mafia birds, who always retaliate. But as mafia birds become more and more common, it becomes safest for host birds to accept every egg. And once most host birds become accepters, it no longer makes sense for cuckoos to spend the energy on retaliation. So mafia birds will become less common—making it safe for hosts to start booting their eggs again. The cycle starts over.
For real birds, things aren’t quite this simple. Cuckoos may lay more than one egg per host nest, for example. Hosts can choose between kicking out a cuckoo egg and abandoning their nest to start a new one. And hosts can change their behavior depending on how many cuckoo eggs they find in their nest.
The researchers put all these factors into a computer model and ran a large number of simulations to see how evolution would play out. Once again, a cyclical pattern emerged. When hosts wait for mafia-style retaliation before accepting a cuckoo egg, mafia birds have an advantage. Then, as the threat becomes more convincing and host birds begin to accept all eggs, cuckoos no longer benefit from their mafia activities.
The frequencies of mafia and non-mafia birds, blind accepters and conditional accepters, swung back and forth in an endless cycle. And although they could lay any number of eggs, cuckoos in the simulations usually evolved to lay only one egg per host nest—just as they usually do in the wild.
The computer model doesn’t prove that mafia behavior is driving the interactions between cuckoos and their hosts in real life. But it fits with scientists’ observations that mafia-style retaliations can happen. The model also shows that the existence of mafia birds could be enough to create the balance we see in the wild, where neither bird has a true upper hand over the other. If birds are flexible enough to change their behavior with the circumstances, they’ll do what they can to avoid getting on the wrong side of the mob.
Image: Great spotted cuckoo by Isidro Vila Verde (via Flickr)
Abou Chakra M, Hilbe C, & Traulsen A (2014). Plastic behaviors in hosts promote the emergence of retaliatory parasites. Scientific reports, 4 PMID: 24589512