Humans often rear their children with help from family and friends. But why would such a strategy evolve? What could we possibly get out of rearing somebody else's child? Now, scientists believe that they've unraveled this mystery — at least when it comes to birds. In some species, birds temporarily forgo having chicks so that they can help their family members raise their children. Here's why this "cooperative breeding" strategy might have evolved.
Though cooperative breeding is a seemingly odd behavior, studies suggest that it occurs in approximately 9 percent of bird species. In this type of child rearing, three or more birds will contribute to caring for the young in a nest, providing both food and protection. Most often, these helpers are the offspring from a previous brood — siblings to the chicks in the nest.
"In some cooperative breeders, the helpers are entirely non-relatives, and in some species there is a combination of relatives and non-relatives," said Naomi Langmore, a zoologist at the Australian National University in Canberra, Australia. "In general, the relatives are better helpers."
When a bird decides to become a helper, it loses out on reproducing during that season. But it's not the case that they're purposefully giving up reproduction to help — they usually wind up helping because there are no breeding territories left for them. And for older siblings, there is an actual benefit to cooperative breeding. "They are helping to raise their brothers and sisters, so they are still getting to pass on their genes to the next generation," Langmore told io9.
Research has shown, oddly, that there are global hotspots for cooperative breeding. That is, the behavior seems to be particularly common in Australasia and sub-Saharan Africa. Some scientists have proposed that maybe these cooperative breeders have a common ancestor who first practiced the behavior, and this is why it's concentrated in certain global regions. Other researchers have suggested that the behavior arises because of highly unpredictable environments, while others have suggested the exact opposite.
Another hypothesis proposes that there's some sort of connection between cooperative breeding and brood parasitism. Brood parasites are birds that surreptitiously lay their eggs in another bird's nest — usually targeting the nest of another bird species — and then flit off, forcing the unwitting parent to raise its young. This trick comes at a cost to the foster parents, because they now have one more mouth to feed. Often the parasite nestling, once hatched, will eject all other eggs or hatched chicks from the nest. It's no wonder, then, that these host parents try their best to keep brood parasite eggs out of their nests.
In Langmore's own research, which focuses on brood parasitism, she found that large parenting groups rarely ever get parasitized. From this, it would appear that cooperative breeders form these defensive groups to fend off brood parasitizes, and that there may be a connection between the two behaviors after all.
Langmore and her colleagues decided to investigate further.
To start, the researchers poured over data compiled by BirdLife International to see how the distribution of cooperative breeding compares with the distribution of brood parasitism. "We were quite amazed at how closely they matched," Langmore said. In particular, brood parasitism also appears to be especially common in Australasia and sub-Saharan Africa.
Of course, this correlation doesn't necessarily mean that there's a causal link between the behaviors. For example, maybe there are unpredictable environments that favor cooperative breeding, and these environments also favor brood parasitism. So the team took a closer look at the data. "We wanted to find out if in those hotspots, whether the hosts were more likely to be cooperative breeders than non-cooperative breeders," Langmore said. Indeed, this is exactly what they found.
The global distribution of cooperative breeding (A) and brood parasitism. Via Science.
But why is it that brood parasite hosts are more likely to be cooperative breeders? The researchers thought there might be three non-mutually exclusive processes going on: brood parasites prefer cooperative breeding species because their offspring will get better care in those nests; cooperative breeders are better able to defend against brood parasites; and cooperative breeding nests are more conspicuous, and thus attractive, because of the increased activity by multiple adults.
To test these ideas, the team conducted 6 years of field observations of the superb fairy-wren, Malurus cyaneus, and its parasite, the Horsfield's bronze cuckoo, Chalcites basalis. They chose these animals in part because the superb fairy-wren is not a 100 percent cooperative breeder — some pairs breed alone, while others are assisted by up to six helpers.
The researchers first tested if the cuckoos had any sort of advantage by parasitizing the cooperative breeders. They found that the cuckoo's chicks grew faster when they were raised by groups of three or more fairy-wrens. What's more, in the cooperative breeding nests, the cuckoo nestlings had a greater chance of surviving to fledglings because the rates of predation dropped with increasing group size. Also, fairy-wrens are known to kick out imposter eggs if they're able to detect them, but cuckoo eggs weren't ejected more often in the larger groups. These results suggest that brood parasites are better off targeting species that are cooperative breeders because their chicks get better care and protection from predators.
However, the researchers also found that these benefits were rarely realized because the nests of cooperative breeding fairy-wrens didn't get parasitized as much as the non-cooperative birds — the larger groups were better able to chase away the cuckoos before they could lay their eggs. Overall, larger groups were more vigilant around their nests and spent more time mobbing the cuckoos than smaller groups, helping to decrease parasitism rates. They also found that the fairy-wrens are more aggressive towards fake cuckoos than they are towards adult and nestling predators, such as Eurasian sparrowhawks and pied currawongs.
The team didn't find any support for their third hypothesis (cooperative breeding nests are more obvious targets), though this didn't surprise Langmore. "At the time when the cuckoo needs to lay its eggs, the only bird going to the nest is the female," Langmore explained. "The rest of the group goes nowhere near the nest, so there are no extra cues for the cuckoos."
At this point, the researchers cannot say what came first: Cooperative breeding or brood parasitism. There's a clear link between cooperative breeding and brood parasitism, but that doesn't mean one caused the other to develop. At the very least, cooperative breeding persists because of brood parasitism. In a perspective article published along with the study in the journal Science, University of Cambridge zoologist Claire Spottiswoode notes that the research shows that the two behaviors "reciprocally influence one another":
Cooperators might be more attractive targets because they make better foster parents, but once exploited by parasites, they are also better able to fight back, helping cooperation to persist.
Top image: A cooperatively breeding speckled warbler (left) feeding a black-eared cuckoo fledgling (right). Courtesy of David Cook.