Where'd I leave that again? —
For small birds, remembering where the food is beats forgetting when it's gone.
It seems like common sense that being smart should increase the chances of survival in wild animals. Yet for a long time, scientists couldn’t demonstrate that because it was unclear how to tell exactly if a lion or a crocodile or a mountain chickadee was actually smart or not. Our best shots, so far, were looking at indirect metrics like brain size or doing lab tests of various cognitive skills such as reversal learning, an ability that can help an animal adapt to a changing environment.
But a new, large-scale study on wild mountain chickadees, led by Joseph Welklin, an evolutionary biologist at the University of Nevada, showed that neither brain size nor reversal learning skills were correlated with survival. What mattered most for chickadees, small birds that save stashes of food, was simply remembering where they cached all their food. A chickadee didn’t need to be a genius to survive; it just needed to be good at its job.
Testing bird brains
“Chickadees cache one food item in one location, and they do this across a big area. They can have tens of thousands of caches. They do this in the fall and then, in the winter, they use a special kind of spatial memory to find those caches and retrieve the food. They are little birds, weight is like 12 grams, and they need to eat almost all the time. If they don’t eat for a few hours, they die,” explains Vladimir Pravosudov, an ornithologist at the University of Nevada and senior co-author of the study.
The team chose the chickadees to study the impact cognitive skills had on survival because the failure to find their caches was their most common cause of death. This way, the team hoped, the impact of other factors like predation or disease would be minimized.
First, however, Welklin and his colleagues had to come up with a way to test cognitive skills in a fairly large population of chickadees. They did it by placing a metal square with two smart feeders attached to each side among the trees where the chickadees lived. “The feeders were equipped with RFID receivers that recognized the signal whenever a chickadee, previously marked with a microchip-fitted leg band, landed near them and opened the doors to dispense a single seed,” Welklin says. After a few days spent getting the chickadees familiar with the door-opening mechanism, the team started running tests.
The first task was aimed at testing how good different chickadees were at their most important job: associating a location with food and remembering where it was. To this end, each of the 227 chickadees participating in the study was assigned just one feeder that opened when they landed on it; all the other feeders remained closed. A chickadee’s performance was measured by the number of trials it needed to figure out which feeder would serve it, and how many errors (landings on the wrong feeders) it made over four days. “If you were to find the right feeder at random, it should take you 3.5 trials on average. All the birds learned and performed way better than chance,” Pravosudov says.
The second task was meant to test reversal learning skills, widely considered the best predictor of survival. Once the chickadees learned the location of the reward-dispensing feeders, the locations were changed. The goal was to see how fast the birds would adapt to this change.
Once the results of both tests were in, the team monitored the birds using their microchip bands, catching them and changing the bands every year, for over six years. “Part of the reason that’s never been done in the past is just because it takes so much work,” says Welklin. But the work paid off in the end.
Remembering to survive
As far as the spatial skills and memory that were tested in the first task were concerned, the best performing chickadee scored close to zero errors per trial over the first 20 trials. It locked onto the right feeder quickly and memorized its location very well. The score of the worst chickadee was close to 2.5 errors. “In most cases the difference wasn’t huge—it was like a one error difference. But it made a huge difference when it came to survival,” Pravosudov said. Chickadees live 2.1 years on average. The best performing birds bumped this average up to 3.1 years. The average lifespan among the worst performing chickadees was just 1.2 years, meaning most of them did not make it through their first winter.
The results in reversal learning, on the other hand, were not correlated with survival nor with the results in spatial skills and memory tests. Chickadees that performed poorly in the first test were equally likely to get excellent or horrible results in the second. “In our previous studies, we learned that spatial skill and memory in chickadees are highly heritable. There is a strong genetic signal and repeatability in this area. We don’t see such genetic signal for reversal learning. For them, being flexible is not as important as maintaining those location memories for as long as possible,” says Pravosudov.
In addition to a longer life and abundance of readily available seeds, there is one more evolutionary reward chickadees got for good spatial memory: more offspring.
The survivors’ legacy
Most chickadees mate for life and breed once a year, with an average clutch of seven eggs. This meant individuals with high scores in spatial learning and memory tests could produce twice as many offspring as the lowest scorers simply because they lived twice as long on average.
But longevity didn’t explain everything. “During one of our studies, we found that when a female chickadee was mated with a male that had better cognition, she laid larger clutches and produced more offspring compared to a female mated with a male with worse cognition,” Pravosudov says. How might females tell whether their mate has good cognition or not? “That’s a million-dollar question. We’re working on this,” Pravosudov adds.
He said the ultimate goal of this project is understanding the evolution of cognitive abilities and natural selection mechanisms. “We’re interested in the dynamics between the flexibility and memory and learning. That’s where we're trying to understand how the reversal learning fits in, because we do know that is important, too, when the birds find new sources of food or when they need to quickly shift,” Pravosudov says.
He argued that this is why developing methods of studying spatial cognition in large numbers of wild birds was so important. Here, it enabled his team to see the impact different traits and skills had on survival. And you can’t test for real-world survival in a lab. “We need this to learn how evolution, genetics, fitness, environment, cognition, all fit into this one big puzzle,” Pravosudov said.
Science, 2024. DOI: 10.1126/science.adn5633