The Elephantine Memories of Food-Caching Birds

A while ago, I searched for a beard trimmer in my bedroom. I spent probably forty-five minutes looking in every likely location at least twice, and every unlikely location at least once. I swore up a storm; the trimmer never turned up. I’ve played similar games with pants. There’s a reason for the burgeoning market in electronic tags that track your belongings.

Our poor memories can seem mystifying, especially when you consider animals. This time of year, many species collect and cache food to stave off winter starvation, sometimes from pilfering competitors. So-called larder hoarders typically keep their troves in a single location: last year, a California exterminator found seven hundred pounds of acorns in a client’s wall deposited there by woodpeckers. In contrast, scatter hoarders—including some chickadees, jays, tits, titmice, nuthatches, and nutcrackers—distribute what they gather over a wide area. Grey squirrels use smell to help them find their buried acorns. But many scatter hoarders rely largely on spatial memory.

People first noticed scatter hoarding by 1720 or even earlier. It’s come under serious investigation, however, only in the past century. Scientists now know that birds’ brains can contain elephantine powers of recollection. Some birds can store, or cache, tens or even hundreds of thousands of morsels in trees, or in or on the ground, and retrieve a good portion of them. In 1951, a Swedish ornithologist named P. O. Swanberg reported on Eurasian nutcrackers: over the course of a single autumn, he saw each bird make some eight thousand caches. That winter, the birds dug through the snow to retrieve their stored food. Swanberg examined the excavation holes the birds had left behind and found nutshells in nearly ninety per cent of them—an indication that there had been few fruitless efforts.

In the late nineteen-seventies, researchers at Oxford buried sunflower seeds just ten centimetres from where marsh tits had buried their own morsels. Over the following days, the bird-buried seeds disappeared significantly faster than those the scientists had buried, suggesting that the birds had precise memories of their cache locations. In 1992, other scientists reported that birds known as Clark’s nutcrackers could recall, with better-than-chance accuracy, where they’d buried seeds more than nine months earlier.

Vladimir Pravosudov began studying food caching as an undergraduate at Leningrad (now St. Petersburg) State University. “I’m a big believer in just watching animals,” he told me. Above the Arctic Circle, he’d spend hours a day with binoculars and a stopwatch, observing willow and Siberian tits; he found that they could cache food as frequently as twice a minute. Extrapolating, he estimated that they could store as many as half a million bits of food each year. He grew fascinated with the question of how and why birds had evolved to be “caching machines.”

Today, as a biologist at the University of Nevada, Reno, Pravosudov focusses on chickadees. It’s impossible to measure a wild bird’s retrieval accuracy precisely: among other things, there’s no way of knowing when a bird looks in the wrong place before the right one. Still, “every time I see them recovering, they look very purposeful,” Pravosudov said. When a chickadee retrieves a seed from bark or lichen, he noted, “They don’t search, they just go boom, and they just pull it out. When you see this, it’s very, very impressive.”

All of which raises a question: If a bird can remember where it’s placed thousands of seeds in the forest, why can’t I find my pants? It would be good to know how birds evolved such extraordinary memories so different from our own.

To find out how natural selection may have forged birds’ brains into steel traps, Pravosudov uses a unique piece of equipment called a feeder array, which his lab installs in the Sierra Nevada mountains. Each array comprises a square frame, about four feet per side, which holds eight automated feeders. Pravosudov’s team catches chickadees in nets and straps tiny electronic tags to their legs; they then program the array so that only one feeder opens for each bird. Once a chickadee discovers its assigned feeder, the system can count how many incorrect feeders it tries per visit before hitting upon the correct one. The birds that make fewer mistakes are learning faster.

The researchers make regular trips into the mountain range on snowmobiles, to maintain the arrays; they use pulleys to suspend the apparatuses from trees in order to protect the birds from bears and squirrels; they adjust the height as snow accumulates under animals’ paws. They’ve studied about a hundred or two hundred birds a year for the past nine years, and have found that the learning ability of first-year chickadees predicts whether the birds will live through their first winter, which is “a big bottleneck for survival,” Pravosudov said. “Even small differences matter.” In a study that was featured on the cover of Science, this past September, the researchers showed that the worst-performing birds live for about a year on average, while the best performers survive to the age of three.

Memory appears to be shaped not only by natural selection—survival of the fittest—but also by sexual selection. Near the feeder arrays, Pravosudov’s lab maintains more than three hundred and fifty nest boxes, where researchers can monitor chickadee mating and reproduction. In a study published in 2019, they found that, when females mated with males who had aced the memory test, they laid more eggs and raised larger broods than when they mated with those who had fumbled—a sign that the females were investing more in the reproductive encounter. “They’re somehow able to know something about the cognitive abilities of their male,” Carrie Branch, the paper’s primary author, who is now a professor at Western University, in Ontario, told me. Branch thinks that the female birds pick up on a signal—perhaps song complexity, or plumage color—which correlates with cognitive ability. (The paper is titled “Smart Is the New Sexy.”)

Ultimately, food caching relies on spatial memory; this, in turn, depends on genes and the brain. Pravosudov has explored the genetic and brain mechanisms on which food caching depends. Research going back to at least 1966 has shown that damage to the hippocampus—a part of the brain involved in memory—hinders birds from returning to their caches. In the nineteen-eighties, scientists found that food-caching species have larger hippocampi than other species. Pravosudov has extended these findings to incorporate the evolutionary pressure exerted by different environments. In 2002, he and his postdoctoral adviser, the comparative-cognitive scientist Nicola Clayton, compared black-capped chickadees captured in Alaska with birds from Colorado, where winters are slightly less harsh. In the lab, the Alaskan birds outperformed the Coloradans, and their hippocampi were disproportionately larger, and contained more neurons. In later studies, he and a postdoc, Timothy Roth, found similar neural patterns in birds from ten North American locations with climates of varying harshness.

London taxi-drivers, who must memorize complex navigational routes, experience growth in areas of their hippocampi. That’s a matter of training, not genes: their brains have bulked up from use, like biceps. To better understand the significance of nature and nurture, Roth, Pravosudov, and their collaborators hand-raised black-capped chickadees, which they’d plucked from nests in Alaska and Kansas. As before, Alaskan birds outperformed comparison birds on spatial-memory tasks; their hippocampi were the same size but the Alaskan birds had more hippocampal neurons. In 2022, Branch, Pravosudov, and their colleagues managed to identify more than two hundred genes associated with spatial cognitive performance, several of which were known to influence hippocampal development. Both genes and nurture, in short, played a role.

The large-scale goal of all this work has been to demonstrate natural selection in action—a rare feat, given the time scales on which evolution happens. Such a demonstration, Pravosudov told me, requires three interlocking elements: scientists must identify variation in the wild, understand the trait or behavior’s genetic basis, and show that it has consequences for survival or reproduction. Pravosudov believes that his lab is the only one currently capable of completing the jigsaw puzzle for cognitive, rather than physical, traits—not just for birds but for any animal. For this kind of work, he explained, chickadees in the Sierras are close to ideal: “They’re not migratory, they don’t go anywhere.” That allows for special access. “We measure them every year. We know everything about their life. We know about their cognition, who they mate with, how many offspring they produce.” Food-caching birds are “a beautiful model to study,” he said, “because food caching puts such enormous pressure on their cognition.”