Acorn Woodpeckers Help Each Other in Times of Plenty

To study the behavior of Acorn Woodpeckers in northern California, one must study the acorns. Walter Koenig, senior scientist at the Cornell Lab of Ornithology, has been investigating these unusual, communally breeding woodpeckers for more than 30 years, and he’s still turning up surprises about why some birds forgo breeding on their own and choose to help others raise their young.

Acorn Woodpeckers live in groups of up to 15 individuals, but only a handful of these actually breed. The others help group survival by tending nestlings and storing thousands of acorns in granary trees for the winter. Because of complicated relationships in a colony—up to seven males compete for the affections of two or three females—the helpers aid in raising their own siblings or half-siblings.

In a recent paper in American Naturalist, Koenig and his colleagues described an unexpected finding: the helper woodpeckers seem to improve the group’s breeding success most when the acorn crop is good, and less when it is poor. Previously, scientists had found that helpers in other species provide more benefit when times are difficult, gathering more food when it is harder to come by. Koenig’s result is surprising because the groups clearly benefit from helpers’ aid when acorns are plentiful.

“A lot more birds are born in good years than bad years,” Koenig said. When there’s plenty of food to go around, the woodpeckers benefit by having lots of birds to store food and later feed it to the young. But in lean years, the group doesn’t do as well because helpers eat what scarce food is present, leaving less for future reproduction efforts by the group.

The yearly crop of acorns is highly unpredictable, so to determine a good crop year from a bad one, the acorns themselves had to be tallied. “We had to go do it ourselves,” said Koenig, who has conducted an acorn survey every year since 1980. Acorns may not have been what the scientists originally expected to study, but the accumulated numbers have shed a great deal of light on their real research subjects.

Originally published in the Autumn 2011 issue of BirdScope.