One of the most interesting, if not the oddest, birds in the Rogue Valley is the acorn woodpecker. I'll explain in a moment, but a little background first.
Most birds are monogamous — at least socially monogamous. Fewer birds, including most grouse, pheasants and relatives, are polygynous, meaning one male mates with multiple females. The opposite, where one female breeds with several males, is called polyandry, and it's rare among birds.
The breeding system of acorn woodpeckers differs from that of most other birds. Their mating system is rather strange, and it all goes back to their diet of acorns. By storing a year's supply of acorns in the fall in a tree called a granary, they must guard the hoard for the rest of the year. To protect the store from jays and squirrels, they need the teamwork of a group usually composed of multiple males, multiple females and young from previous nesting seasons. However, this creates a little tension when it comes time to raise a family. Who in the group gets to breed? I do! I do! I DO!! I do!? You see the problem. If it sounds complicated, the name given to this special living arrangement is even worse. It is called polygynandry. Literally this means "many females and many males."
The younger birds in the group get to sit out this little problem. This still leaves multiple males and females that wish to breed. Naturally, each wants to be the genetic parent of all the young. This is how the game in nature is played. The one with the most young wins. For males it's an easier game, though with less-certain results. They mate with all the receptive females and hope for the best.
The rules are a bit different for females. The group has a single nest, and each female wants her eggs to be the only eggs in the nest.
If a female enters the nest and finds an egg, but she hasn't laid any of her own yet, she knows it isn't hers. The answer is simple. Out goes the egg, often scrambled on the ground. The first female, watching all this, removes the egg of the second female from the nest and makes another scrambled egg. This goes on until they get confused about whose egg is whose. Once muddled, they complete the clutch and settle down to incubate the eggs in peace. There are no issues from there on out. Each adult in the group is now convinced and content that they are the proud genetic parents of all the young in the nest.
Researchers recently have suggested a less intriguing explanation. Egg tossing may actually be a way to synchronize breeding. By delaying the egg-laying of one female until the second is ready to lay her eggs, they ensure the eggs all hatch at the same time.
Either way, acorn woodpeckers are oddballs.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.