Male acorn woodpeckers that share mates with their brothers live longer lives, have better quality homes and father more baby woodpeckers than those that choose a monogamous lifestyle.
Most acorn woodpeckers (Melanerpes formicivorus) form lifelong partnerships with a single mate, but about a third of females and half of males opt for breeding in sibling groups, sharing one or more mates with their sisters or brothers. 51¶ŻÂţs used to think males in these groups were trading their chance of paternity for comfort – gaining access to bigger, better and safer nests – but now it seems they are actually getting the best of both worlds, says at the Smithsonian National Museum of Natural History in Washington DC.
“We don’t know how much energy they’re saving and how much they’re reducing their stress, but we know these cooperative breeders usually live in higher quality territories with more stored food, which may potentially increase their lifespan and thus their breeding attempts, leading to more total chicks,” says Barve.
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These better territories – oak trees ready-equipped with thousands of acorns stashed in holes drilled by woodpeckers that lived there before – also tempt young adult offspring to stay in the nest longer as non-breeding helpers, ostensibly reducing the parents’ workload and hence stress levels. “If you’re living in a mansion, you’re going to take longer to move out,” he says.
Researchers at Hastings Natural History Reservation in California have been gathering data from free-ranging acorn woodpeckers since 1968 and collecting blood samples from each bird since 1984. The work has allowed them to compile genetic information on thousands of birds over dozens of generations, including many living in polygynandrous communities in which both males and females share partners with their same-sex siblings within stable, lifelong groups.
Analysing more than 50 years of data, Barve and his colleagues were “definitely surprised” when they realised that the co-breeding males actually fathered 50 per cent more chicks than the monogamous ones, he says. “For the longest time, we thought these single-breeding males were making the most number of chicks [because they father every egg the female lays], but that’s not true,” he says.
“And this is the really cool part,” he says, “because it’s all about having a longer reproductive lifespan. You might have a single-breeding male that’s alive for four years and has 12 chicks during that time, but a male co-breeding with another male might live seven years and end up leaving 18 chicks.”
As for the females, they have about as many chicks over their lifetime whether they breed alone or in a two-sister group, says Barve. If they breed as three sisters, though, their total chick count actually drops. Females usually lay four eggs at a time, but in shared nests, females that haven’t laid eggs yet tend to toss out their sisters’ eggs. Not surprisingly, then, most of the co-breeding groups include only one or two females, he says.
What exactly makes these group-breeding birds live longer merits further study, says Barve. He suspects it is related to their ability to team up to fight other birds for better homes with more stored acorns for lean seasons, reduced stress due to shared workloads and more eyes and ears scanning for hawks and other predators.
Proceedings of the Royal Society B
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