Long-Term Field Studies Explain Why These American Woodpeckers Are Communists | @GrrlScientist
Combining genetics with many decades of field studies is providing a surprising answer to one of evolutionary biology’s biggest questions: why cooperate?
Acorn woodpeckers, Melanerpes formicivorus, are medium-sized woodpeckers with shiny, iridescent black plumage, creamy white underparts and white facial markings, a gleaming scarlet cap and pale golden eyes. These outgoing birds were named for their dependence upon acorns. Since acorns only appear seasonally, these birds ensure a steady supply throughout the year by drilling holes in trees (or telephone poles, or fences, or houses, or any wood structure) for storing them. These so-called “granaries” are typically enormous — often consisting of thousands or even tens of thousands of individual holes.
But these handsome birds’ dependence upon acorns isn’t the only life history trait that makes them unique: acorn woodpeckers have a complex social structure that sets them apart from all other birds. Acorn woodpeckers live in family groups that can number up to 15 adults, all of whom maintain and defend the family granary and help raise chicks that come from just one nest. But this is an unusual family structure amongst birds — so unusual that Frank Leach, the scientist who first noted it, published a paper in 1925 declaring that these birds were practicing communism (ref).
In that paper, observations from long-term field studies suggested that acorn woodpecker communities’ unique behavior and demography result from patterns of acorn production and from the geographic distribution of oak trees. Basically, if acorn woodpeckers weren’t exclusively dependent upon eating acorn nuts, which are only seasonally available, these birds’ social behaviors would not have arisen at all.
Acorn woodpeckers’ behaviors are so interesting that scientific field studies have been going strong for decades. For example, for more than 40 summers, ornithologist Walter Koenig, now a professor emeritus at Cornell University in New York, and his research team have travelled to Hastings Natural History Reservation in California, so they can continue their studies of these birds’ complex social structure.
Every year, Professor Koenig and his collaborators capture hundreds of woodpecker chicks and mark them with color-coded leg bands so individuals can later be identified from a distance and tracked using binoculars and telescopes.
More recently, Professor Koenig and his collaborators have been collecting DNA from their study birds, so now they can track genetic relatedness between individuals and use these data to gain insights into one of evolutionary biology’s most fundamental questions: why cooperate?
Genetics have revealed that acorn woodpeckers are polygynandrous, an extremely rare “communal” behavior in birds where multiple males and multiple females breed and attend each nest. Research has shown that as few as two or as many as 15 acorn woodpeckers attend any one nest, and there can be as many as five co-breeding males and as many as three co-nesting females. But are these birds related to each other?
“The breeder males are all related to one another, the breeder females are related to one another, and their offspring are, of course, related to both,” Professor Koenig explained. “But the breeder males are not related to the breeder females,” so the birds avoid inbreeding.
The genetic data also revealed that some adults attending these nests were not breeding at all.
“About a third of the population at any one time is made up of ‘helpers’ who are still living in the group in which they were born, and have not yet found a way to breed on their own,” Professor Koenig said. “So instead, they’re staying there, helping to raise younger siblings.”
In evolutionary terms, individuals are only successful if they pass their genes on to the next generation, so why would a young woodpecker sacrifice its own reproductive success by helping its parents raise more offspring?
The answer lies in the granaries themselves; a limited resource that are essential to acorn woodpeckers’ survival but require tremendous amounts of time and effort to construct, maintain and defend. Granaries consist of thousands of individual compartments, each of which contains an individual acorn. Each hole takes an average of 20 minutes to drill, and is painstakingly sculpted to hold a particular acorn. Starting a new family group requires starting a new granary, so leaving the parental group is a serious decision.
“They prefer to be breeding if they could, but since they can’t, the next best thing is to stay in the group in which they were born and help raise younger siblings to which they are related,” Professor Koenig said.
Since the young birds share half their genes with each parent, on average, and also with their siblings, on average, helping their parents raise more offspring is the next best thing, in an evolutionarily sense, to leaving and raising their own offspring. By helping their parents, the young birds are helping themselves, too.
If Frank Leach hadn’t conducted his own long-term studies almost 100 years ago that led him to notice that acorn woodpeckers are “communists”, we wouldn’t have even known we could look to birds and ask the important question: why cooperate? Then, without additional data from subsequent long-term studies, we would never have known for sure who was related to whom, which reveals the genetic and evolutionary bases for why cooperation happens at all.
Walter D. Koenig and Eric L. Walters (2016). Provisioning patterns in the cooperatively breeding acorn woodpecker: does feeding behaviour serve as a signal? Animal Behaviour, 119:125e134 | doi:10.1016/j.anbehav.2016.06.002
Walter D. Koenig and Eric L. Walters (2015). Temporal variability and cooperative breeding: testing the bet-hedging hypothesis in the acorn woodpecker, Proceedings of the Royal Society of London B: Biological Sciences, 282:20151742 | doi:10.1098/rspb.2015.1742
Walter D. Koenig, Eric L. Walters, Johannes M.H. Knops, and William J. Carmen (2015). Acorns and Acorn Woodpeckers: Ups and Downs in a Long-Term Relationship, Proceedings of the 7th California Oak Symposium: Managing Oak Woodlands in a Dynamic World, 23–33 | via ResearchGate
Frank A. Leach (1925). Communism in the California woodpecker, The Condor, 27(1):12–19 | free PDF
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Originally published at Forbes on 18 October 2017.