One sunny day in California’s Sierra Nevada Mountains, Diana Tomback met her first Clark’s Nutcracker. As she rested under a pine tree, she noticed a bird wrenching scales from a pinecone. Its persistence in stabbing the cone with its bill, peeling off the scales, and extracting individual pine seeds mesmerized her. She had to know more, and when a park ranger ambled by, Tomback demanded information. That bird, the ranger said, was a “pine crow,” and the tree was a whitebark pine.
When she returned to graduate school, she was determined to find out more. “I was utterly consumed by this bird, its behaviors, and the interaction with the whitebark pine,” Tomback says. A bit of research revealed this ‘pine crow’ to be a Clark’s Nutcracker. But she couldn’t find much else. The Eurasian Nutcracker had been studied in Russia, Germany, and France, but there was little known about its North American cousin.
“I realized: My gosh, what an incredible bird and interaction to study,” she says. So Tomback devoted herself to the cause. Since the 1970s, Tomback, who’s now an ecologist at the University of Colorado, Denver, has published dozens of papers about the Clark’s Nutcracker. Along the way, she's discovered that the birds are more remarkable than she imagined. Clark’s Nutcrackers are winged foresters, whose penchant for hoarding seeds contributes to the growth of new pine forest.
A Craving for Cones
Clark’s Nutcrackers are experts at the difficult art of freeing seeds from pinecones. Each cone contains dozens of seeds, which are inaccessible to most animals until autumn when the cones open and spread their protective scales. But nutcrackers don’t have to wait. As stiff, unripe cones become available each July, they jab their strong, piercing bills between scales to loosen and tear out bits of seeds.
It’s not easy work. Clark’s Nutcrackers spend so much time hacking apart pinecones that sticky resin sometimes dyes their grey feathers a reddish-purple color. But the prize is worth the effort: Pine seeds are a nutritious food, packed with fats, proteins, and carbs. They are so rich that it doesn't take many to satiate a nutcracker's appetite—and it's then that the bird’s habits get really interesting.
Photo: Marshal Hedin/Flickr CC (BY- SA 2.0)
Hide and Seek
While other birds prepare for winter by feasting and fattening up for lean times, these sleek gray corvids use their landscape as a larder. After a Clark’s Nutcracker eats its fill of pine seeds, it stores the rest—upwards of 100 pine seeds at a time—in an expandable pocket below its tongue.
The bird then flies around the forest, burying clusters of four or five seeds in the soil; during peak pinecone season, it will cache up to 500 seeds per hour. By the end of the fall, each nutcracker has stashed tens of thousands of seeds, a food source it relies on throughout the winter.
Remarkably, the birds manage to find their caches later. They often hide seeds near the base of tree trunks, a tendency “that may play an important role in their spatial memory system,” Tomback says. Landmarks help nutcrackers remember the precise locations of caches, so they can retrieve and eat seeds when trees are coneless and the weather turns cold.
“These birds probably remember up to 10,000 caching locations” at a given time, says ecologist Mario Pesendorfer of the Cornell Lab of Ornithology. But their memory fades over time. Within nine or 10 months, many uneaten seeds lie forgotten beneath the forest floor.
Foresters with Wings
Nutcrackers often bury their seeds at the perfect depth for germination, and given time and a bit of luck, abandoned seeds will sprout and grow into new trees. In this way, their overzealous seed hiding means that Clark’s Nutcrackers play a critical role as foresters, planting new generations of pine trees that conveniently spring up from the oversupply.
Crucially, the birds will hide seeds as far as 20 miles away from their source trees. In doing so, they help trees expand their territory into new areas. As development continues to fragment forests and climate change demands rapid migration, “animals that move between patches of habitat are increasing in importance,” Pesendorfer says.
Nutcrackers have an especially close relationship with the whitebark pine. Even in the fall, whitebark pinecones don’t open on their own; the seeds remain trapped behind tightly locked scales. It’s largely up to Clark’s Nutcrackers to free those seeds with their hammering beaks and then help spread them. Also unlike other pines, whitebark seeds don't have "wings" that let them ride gusts of wind across the landscape. Instead, the seeds and cones seem optimized for a nutcracker's bill, and as such, the trees rely on the birds’ forgetfulness to reproduce.
The relationship has served both species well, but researchers are growing worried because whitebark pines are now in decline. A deadly fungus called white pine blister rust is spreading through the West's pine forests. Simultaneously, mountain pine beetles are chewing through the trees, a problem made worse by climate change. The loss of pinecones shouldn't threaten Clark's Nutcrackers; they can feed on a variety of seeds and cones along with insects and fruits, so they are safe for now. But as whitebark pines vanish from the landscape, they will produce fewer seeds, and the birds that once helped them proliferate might not be able to cache enough to sustain the tree.
However, it's possible that the birds could play a part in helping the whitebark pine recover. Human foresters are now studying how to attract Clark's Nutcrackers to aid in forest restoration. As fantastical as it sounds, this isn’t an unprecedented effort. Since at least the 1950s, German foresters have harnessed the abilities of another scatter-hoarding corvid, the Eurasian Jay, to their advantage. Instead of doing the hard labor of replanting oak trees, they put out buckets of acorns and “let the jays do their thing,” Pesendorfer says. It's estimated that the jays plant up to 1,600 oaks per acre. If Clark's Nutcrackers could do the same—but with whitebark pine seeds resistent to blister rust—they could be the savior's of the West's pine forests.