Featured

Some Birds Have Babysitters – but Why Are Babysitters Available in the First Place?

Surrogate parents of unrelated nestlings could forgo food while incubating eggs and foraging for chicks, and they lose weight. What makes them act in such a generous manner? Are they altruistic?

A surrogate parent feeding a Seychelles warbler chick. Credit: Sjouke Kingma

A surrogate parent feeding a Seychelles warbler chick. Credit: Sjouke Kingma

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Young male moustached warblers (Acrocephalus melanopogon) are the saviours of single mothers. If the females’ consorts disappear, these bachelors step in – incubating eggs, feeding chicks and chasing predators away. Why do they forgo their chance of breeding and help to raise other males’ young?

These warblers are by no means exceptional. Other species like fairy-wrens (Malurus spp.) live in groups of relatives and unrelated individuals. Many die so the surviving spouses hook up with others, so it is inevitable for the babysitters to raise unrelated young.

At least bell miners (Manorina melanophrys) assist their own family and relatives. Instead of leaving their parents’ territory after fledging, the young adults stay to help. Aiding one’s own kin – called kin selection – benefits parents and siblings. Parents don’t have to work as hard, and with the energy savings, they live to breed another day. The young don’t lack for anything with the extra attention. Solicitude is a trait in birds that breed in cooperative groups, which make up about 9% of all bird species.

At one time, ornithologists assumed all helpers were related to the breeders, but molecular testing revealed that strangers regularly played surrogate parents in about 20% of these cooperative nesting birds.

Evolutionary biologist Christina Riehl, then at Harvard University and now at Princeton University, New Jersey, reviewed how and why these seemingly altruistic birds provide their services to breeding adults.

While playing nanny, young male pied kingfishers (Ceryle rudis) try to sneakily mate with the breeding female. If that ploy fails, they hang around as helpers, hoping to father the next round of offspring. The female is more likely to accept a helper as mate if he assists with her previous batch of young.

Need for territory

But not all surrogate parents of unrelated nestlings gain such clear payoffs. In fact, they could pay a price. The helpers forgo food while incubating eggs and foraging for chicks, and they lose weight and body condition. This could mean they aren’t in the best shape to reproduce and they could even jeopardise their own survival. What makes them act in such a generous manner? Are they altruistic?

According to one theory, in return for being allowed into the breeders’ territory, parental aides ‘pay rent’ by helping with childcare. Another says helping others allows them to practice skills they’d need later in life. Yet another suggests birds are programmed to stuff food down the gaping mouths of any chicks, whether related or not. Other inferences include shortage of females and formation of alliances.

Evolutionary biologist Sjouke Kingma of the University of Groningen, the Netherlands, has worked on cooperative breeding in birds since 2010. To find an overarching theory that explained surrogacy, he analysed 44 species of birds that provided parental assistance to others of their kind.

Birds need to have territories to meet all their needs – food, water, shelter and breeding sites. For 28 species that he examined, territories were at a premium. Where habitat is completely carved up by others, about 30% of the subordinates inherit their fiefdoms in part or whole from breeders. Could territory be the key reason for providing nestling care?

“I got this idea from a study on fairy-wrens,” Kingma told The Wire. “The idea is not so new.”

The remaining 16 species don’t have real estate concerns. Non-breeders of this group help others in the colony if they themselves fail to breed or they haven’t matured yet. But the majority have an asset problem. When this resource is in short supply, these species trade child-rearing services for property. This incentive drives parental aides to keep their wards well-fed, overlooking the distinction between relatives and strangers. They work themselves to the bone, striving 50% harder than helpers that don’t suffer a territory crunch.

“Birds see their territory in the same way as we see our house,” Kingma said in a press release. “My research reveals that the ‘home-owners’ get much more help if the helpers stand to inherit their territory in the future. After all, you’d be much more inclined to help someone maintain their home if you thought you’d inherit it one day.”

How would birds know they stand to inherit the territory? How do they strike the deal?

“I think individuals might not know,” replies Kingma. If single birds with no prospects of breeding help and as a result, gain benefits, natural selection chooses that behaviour. “So I don’t know if it works within a species directly, but variation across species may have evolved this way.”

It’s an arrangement that benefits all parties. The parents gain extra help in raising young, and the nestlings get better care and thus survive to adulthood. The surrogate parents get paid in property later with the added bonus of raising a crop of future aides that may help them with their own parental duties when the time comes.

A property trade

Although the idea that birds might trade childcare services for territory is not new, “nobody really showed empirical evidence for it on this scale,” says Kingma.

Benefiting from providing support probably plays an even greater role than is often assumed, says Riehl. “The exciting thing about these results is not that they undermine the importance of kin selection – in fact, they highlight the importance of kin selection in some cases – but that they illustrate the diversity of ways that natural selection can favour cooperative breeding in birds. Kin selection and direct benefits aren’t mutually exclusive hypotheses. Rather, they often reinforce each other, and both are important drivers of cooperation.”

In many species, property titles are dearer to males. It’s possibly no coincidence that the majority of surrogate parents are males. Territory is essential for females of some species. Would they act as au pairs in exchange for property too?

“Males and females would equally benefit,” says Kingma, “although we see that helpers are more often male than female in many, but not all species. I think that the role of kin selection is slightly overestimated. Helping unrelated individuals may bring lots of benefits in the future.”

Indeed, female white-winged trumpeters (Psophia leucoptera) assist a breeding female whose territory they’ll inherit when she dies. Since young females are kicked out of their homes by their parents and cannot establish their own fiefdom, assisting a dominant female is the only way they can take a shot at reproducing.

Kingma thinks many more questions remain to be answered. For instance, what determines which particular helper inherits territory? How is hierarchy established among many helpers? How do they resolve property disputes?

Seen through the prism of real estate shortage, nannies among fairy-wrens and moustached warblers have struck a bargain deal.

The study was published in the journal Nature Communications on October 23, 2017.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.