The common cuckoo is famed for its knack for mooching off the parental instincts of other birds. It lays its eggs in the nests of at least 100 other species, turning them into inadvertent foster parents for its greedy chicks. For this reason, it’s called a brood parasite.
It’s not alone. Among the birds, the full list of brood parasites includes more than 50 members of the cuckoo family, cowbirds, honeyguides, several finches, and at least one duck.
Now, William Feeney from the Australian National University has found that brand of reproductive cheating goes hand in hand with its polar opposite: cooperative breeding, where birds raise their young with help from siblings or offspring, often at the cost of the helpers’ own reproductive success.
The two strategies couldn’t be more different but Feeney found that each drives the evolution of the other. In places where one is common, the other is too. Exploitation goes hand-in-hand with cooperation.
Biologists have been exploring the origins of cooperative breeding for almost 140 years. “What I liked about the new paper is that is presents convincing evidence for an idea that wasn’t even really on the table—it is beneficial because groups reduce the risk of brood parasitism,” says Bruce Lyon from the University of California, Santa Cruz.
Similarly, scientists who study brood parasites have mostly focused on defences like spotting a cuckoo’s eggs. “For some reason the social system of the hosts was not really considered,” says Lyon.
The seeds of the discovery were planted in Australia, where Feeney’s group were studying the aptly named superb fairy-wren. It gets parasitised by Horsfield’s bronze cuckoo, but not without a fight. The team noticed that the fairy-wrens attacked incoming cuckoos with extreme prejudice, and that larger groups almost never got parasitised. They wondered if the two behaviours—cooperative breeding and brood parasitism—were connected in other parts of the world.
The answer was a resounding yes. Look at the maps below. The top one shows the global spread of cooperatively breeding passerines—the small perching birds that are the most frequent target of brood parasites. Red areas are rich in cooperative species, while white areas are devoid of them. The bottom map shows the spread of brood parasites. There’s a very strong match between the two. Even if you account for the total richness of species in a given area, places with lots of cooperative breeders also have lots of brood parasites. Africa and Australasia are particularly rich in both.
Of course, this connection could be due to some unrelated factor. For example, extreme parenting styles might just be more common in Africa and Australia, because these continents have harsh, variable environments. If the two strategies really are connected, then you’d expect that connection to hold within regions as well as between them.
That’s exactly what Feeney found. Even within Africa and southern Australia, brood parasites are much more likely to target cooperative breeders than other birds.
Here’s a family tree showing all the birds from southern Africa. The orange circles denote species that are cuckoo hosts, and the blue circles are the cooperative breeders. Around 28 percent of the hosts help each other out in the nest, compared to just 8 percent of the non-hosts.
And here are all the passerines in Australia. Again: an obvious connection. Around 53 percent of the hosts help each other out in the nest, compared to just 12 percent of the non-hosts.
There are two possible reasons for this correlation. Brood parasites might selectively target cooperative breeders because they’d provide the best care. Alternatively, birds might resort to teamwork because they can better defend their nest against parasites.
Feeney found that both answers are right.
His team returned to the superb fairy-wren—a bird where some parents get help in the nest, but others don’t. By comparing different nests over 6 years, they showed that cuckoo chicks grow faster and survive better if they’re raised by larger groups of fairy-wrens. So cooperative breeding can foster the rise of brood parasitism.
But cuckoos rarely get to realise this benefit, because larger groups of fairy-wrens are also better at fending them off. Collectively, they’re more vigilant around the nest. If one of them spots a cuckoo, it makes a cuckoo-specific alarm call and the entire group mobs the intruder. The larger the group, the more persistently they attack. So the presence of brood parasites fosters can foster the rise of cooperative breeding.
Naomi Langmore, who led the study, thinks that the relative strength of these two effects probably changes over time. Cuckoos and other brood parasites often switch hosts. When this happens, it’s initially easier for them to reap the benefits of a larger surrogate family because the new hosts have evolved to detect or repel their infiltrators. Over time, as such defences emerge, the balance shifts. For the fairy-wrens and bronze cuckoos, “cooperation protecting against parasitism is the stronger force,” says Langmore.
Of course, there are many reasons for species to evolve cooperative breeding, and the threat of parasites is just one of them. If there aren’t enough territories to go around, or if predators are particularly rampant, it will also benefit youngsters to stay near their families rather than strike out on their own. “Brood parasitism is not exclusive of other factors but may simply help tip the balance in favour of helping over dispersing,” says Lyon.
“The relative importance of brood parasitism in selecting for cooperative breeding is likely to vary from species to species,” says Langmore, “but our evidence suggests that, overall, it is one of the major selective forces favouring the evolution of cooperation.”
She’s not just talking about birds, either. There are also many brood parasites among the insects, including a group of over 3,000 cuckoo waspscuckoo wasps. Many lay their eggs in the nests of other wasps, and their grubs devour the hosts’ own eggs and larvae. “The hosts of cuckoo wasps also mount highly aggressive colony attacks on the parasites,” says Langmore, “and hosts from parasitized populations have actually evolved larger bodies so they are better able to drive off the parasites.”
Reference: Feeney, Medina, Somveille, Heinsohn, Hall, Mulder, Stein, Kilner & Langmore. 2013. Brood Parasitism and the Evolution of Cooperative Breeding in Birds. Science http://dx.doi.org/10.1126/science.1240039