Acorn woodpeckers may be noisy birds with clown faces, but their lives are like Greek tragedies, with communal mating and skirmishes over acorn granaries.
On a sunny day in May, Walter Koenig is balancing atop a 40-foot extension ladder raised straight into the sky in the Hastings Natural History Reservation, 2,350 acres of rolling hills in the Santa Lucia Mountains of Monterey County, California. The base of the ladder is barely lodged in the loose soil of a grassy slope, which is steep enough that I have some trouble keeping my footing. The top of the ladder is leaning against—against nothing, actually.
Moments ago three student assistants helped Koenig pull the ladder upright with four 100-foot ropes, as if raising a circus tent, securing them to four temporary anchors—including the bumper of a truck. Now Koenig, a researcher at the University of California at Berkeley’s Museum of Vertebrate Zoology, is perched within reach of an acorn woodpecker nest cavity on the branch of a valley oak. Usually Koenig relies on mountain-climbing gear to hoist himself up to a nest, but this tree has no limbs close enough to the cavity to hold a rope. In his mid-fifties, Koenig appears as fit and youthful as one might expect of someone who since 1974 has been shinnying up trees to peek into acorn woodpecker nests. His soft features are partly hidden by the turned-down sailor’s cap of blue terry cloth he wears. Koenig knows it looks goofy (“Someone once yelled out a car window at me: ‘Hey, dude, time for a new hat!’ ” he says), but on hot days he can soak it in water to keep his head cool.
At the top of the ladder he pauses for a moment. “Let me think about this,” he says, considering how best to retrieve the baby woodpeckers from the cavity without losing his balance. Soon he is back on the ground holding three nestlings that are all bulging eyes and bristly, half-formed feathers. Each chick has a crew cut that will eventually develop into the red cap an adult wears like a jaunty beret. “Look at these adorable Santa Claus faces,” Koenig says, as he starts fitting the chicks with colored anklets and taking blood samples. When he climbs back up the ladder, Koenig performs the small ceremony he’s been repeating for decades: Holding each bird up momentarily to his face, he says, “Live long and prosper,” then returns the woodpecker to its nest.
If the acorn woodpecker is to continue to prosper as a species, it will be in no small part due to Koenig’s lifelong devotion to understanding the bird’s natural history and how its life relates to the ecology of its habitat, which in many areas is being destroyed or degraded. A noisy, well-known bird of oak and pine-oak woodlands in California and the Southwest, the acorn woodpecker is actually found as far north as Oregon and south through Mexico, Central America, and into northern Colombia.
Its social behavior is unique among North America’s 22 woodpecker species, which typically spend their time alone or paired with a mate. At Hastings, the acorn woodpeckers live in groups of up to 15 birds that remain together all year. Banding young acorn woodpeckers and drawing blood for DNA analysis help Koenig keep track of who is who—and who is whose. Parentage among acorn woodpeckers is a complex affair. In the spring mature males compete for breeding rights with the females in their social group, but rather than form monogamous pairs, the dominant males and females mate with one another seemingly indiscriminately.
Furthermore, all of the birds in the group help care for the young. Such “cooperative breeding,” as this social system is called, is known to occur in only 300 species worldwide. If there is more than one breeding female in an acorn woodpecker group (as is the case 20 percent of the time), the cooperative breeding behavior achieves an incomparable esprit de corps. The two females (sometimes three) will lay their eggs in the same nest cavity and take turns incubating them and raising the chicks when they hatch. “There’s nothing quite like it,” says Koenig. “A few species lay eggs in a communal nest, but none truly share a nest site the way female acorn woodpeckers do.”
When Koenig set out to study the acorn woodpecker more than 30 years ago, little was known beyond the basic facts of its social system. Early on, Koenig’s fieldwork took an inter-esting turn. One morning in 1978 he and colleague Ronald Mumme observed a woodpecker fly past with half an eggshell in its bill. Koenig assumed that chicks had hatched in a nearby nest and the adult was clearing the nest of the shells. But when he climbed the tree and peered into the nest, it was empty. Two years later Mumme saw an adult enter a nest and then leave moments later with a whole egg. It took several more years of intensive observation before Koenig and Mumme understood these two events. Females nesting together were removing eggs from the nest and storing them in a nearby tree. Later, the eggs became a midday snack for any member of the family group, including the females that laid the eggs.
The logical conclusion was that co-nesting females were destroying one another’s eggs to gain a reproductive ad-vantage. But Koenig and Mumme were not satisfied with this speculation. Year after year they continued to check nests, paying particular attention to these “egg demolition derbies,” as they called them. Eventually they determined the females were destroying eggs until both of them began laying eggs synchronously. This ensured that neither was laying her first egg significantly later than the other, which meant that the chicks hatched at about the same time and, thus, would be equally capable of competing for food brought to the nest.
The yin-yang of cooperation and competition in the acorn woodpecker’s life continues to fascinate Koenig. As I followed him about the rolling hills of Hastings it was clear he had lost none of his enthusiasm for his work. When we entered a new woodpecker territory, Koenig called out a half-human, half-woodpecker greeting, heeh-wooo-aaaa, that sounded like an Elmer Fudd “hello.” Gliding up the hills with long strides, he talked about one of the most competitive moments in an acorn woodpecker’s life. If all the mating males or females in a group disappear, leaving mating openings to be filled, a furious power struggle ensues. Not only do birds within the group vie for mating status, but birds from other groups join the fray as well. In these turf wars the birds often form gangs that then fight together against rival gangs. The battles— chases, vocal threats, skirmishes, pecking at one another’s heads—last for days or weeks.
The acorn woodpecker’s social life has no doubt evolved over thousands of years. And one ecological factor appears to be at the heart of it: the lure of prime real estate. Though the woodpeckers feed mainly on insects throughout spring and summer, in the fall they turn their attention to acorns. And here they earn their name. Each group harvests acorns by the thousands, enough in a good year to last them through the winter. While many birds store food, none has a system to rival the acorn woodpecker’s. Each acorn is stored in a pre-drilled hole in a dead tree (or fence post or barn wall or telephone pole). From a few feet away, a good “granary” tree appears to be riveted with acorns.
The granary is joint property, and the birds maintain and defend it year-round with vigilance and bravado. It is the key to their survival through the winter months. Exceptional granaries, which may contain tens of thousands of acorns, are passed on from generation to generation like the family jewels.
The connection between real estate and the woodpecker’s social life becomes even more apparent when the acorn woodpeckers at Hastings are compared with those that have been studied in a population in Arizona, whose birds form monogamous breeding pairs. They regularly migrate and have no granary trees at all. Furthermore, there is no direct evidence in the Arizona population of two females nesting together. “I think it has something to do with not just the quality habitat but also the amount of marginal habitat available,” Koenig says.
He points out that in Arizona and New Mexico there is more habitat of marginal quality—areas where young birds might disperse as juveniles typically do and at least attempt to make a go of it. This pressure to share the limited habitat available may explain why the group social life of the woodpeckers at Hastings evolved.
Lately Koenig and others have been concerned with a threat to the oaks that are the mainstay of quality acorn wood-pecker habitat. About 300 years ago, when some of the oaks at Hastings were young trees, the Spanish who settled California brought various grasses here as cattle feed. The grasses spread quickly, replacing such natives as bunch-grass. Unlike bunchgrass, the introduced grasses have shallow root systems, which, in spring and summer, use up the water in the top few inches of soil. Later in the year, when the acorns drop to the ground, there is little moisture avail-able to help them germinate.
To make matters worse, gophers, burrowing here to there and everywhere in between, essentially turn over the soil as if farming it. And studies have shown that the European grasses grow three times better in “gopher-tilled” soil than do native grasses. Furthermore, these grasses—again unlike the native perennials—drop large seeds by the millions, providing gopher food wherever one of the subterranean architects pops up.
Although it’s hard to know if gopher populations have in-creased since the invasion of the nonnative grasses, it’s clear the gophers are doing well in this altered landscape—pop-ulation densities may reach 2,000 animals per acre. If any acorns somehow manage to germinate in the moisture-depleted soil, these seedlings become gopher food, too.
This may help explain the almost complete lack of regener-ation of valley oaks and blue oaks throughout California, the two species most favored by acorn woodpeckers. Biologists began to notice the absence of oak saplings several years ago and have been studying the alarming trend ever since. If nothing changes, it appears there will be no young blue and valley oaks to take the place of the mature trees when they die.
The loss of these trees is just one of the issues Koenig is following as he continues his long-term study of the acorn woodpecker. Lately he has been perplexed by the activity of woodpeckers in a 200-yard-long stretch of the valley near the Hastings headquarters. “There are five different groups in this area now,” he says, “where there used to be only two groups. Some have almost nothing you could call a granary. I don’t understand that.”
If the Hastings birds formed social groups to protect a shared granary, then why did these groups exist without a granary? “The more you study something, the more you see things change for no apparent reason,” Koenig says. With a grant from the National Science Foundation, he is putting up artificial nest cavities and providing potential granaries in several study sites to see how important these are to woodpeckers as they choose a territory. Koenig, meanwhile, is perfectly happy that there is no end in sight to his research. “I’ll be checking nests the rest of my life,” he says, smiling.
On my last morning at Hastings, I sat a few yards from a gnarled oak where five woodpeckers were clustered in the branches near a nest cavity. The sun was rising over the hills behind them, and the birds were silent, not yet awake, it seemed. Suddenly, one let out a loud waka-waka-waka-waka. Two other birds repeated the call, louder and faster, as if trying to catch up to the rhythm of the first. And then all the birds let loose in a jamboree of overlapping cries that rolled through the air and seemed to follow the dark green of the oaks up into the hills and beyond.
Don Stap, author of Birdsong: A Natural History and A Parrot Without a Name, teaches English at the University of Central Florida.
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State of the Bird
Looks: Black and white, with clown-faced pattern, red cap, staring whitish eyes.
Behavior: Distinctly odd. Lives in colonies, cooperating to store acorns in “granary trees” in fall for consumption later. Populations are cooperative in nesting also, with all adults in a colony helping to incubate the eggs and raise the young in the nest.
Range/habitat: Mostly permanent residents in oak woods of western United States and Mexico, south locally through Central America to northern Colombia.
Status: Surveys show slight increases in population in some areas, declines in others, with overall population more or less steady.
Threats: Loss of habitat could lead to greater isolation of colonies and local extinctions. Certain diseases of oaks, and lack of reproduction by some oak species, are also cause for concern.
Outlook: Probably secure, so long as healthy oak groves can be maintained throughout its range.—Kenn Kaufman
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