Researchers have been fascinated by the black-capped chickadees' remarkable memory and their ability to remember the locations of thousands of stored food items. Dmitriy Aronov, a neuroscientist at the Zuckerman Mind Brain Behavior Institute at Columbia University, recently visited Duke University to discuss the challenges of studying wild birds in a lab and his team's investigations into how neural activity represents the formation and retrieval of episodic memories in black-capped chickadees. Aronov's research showed that every moment of caching creates a lasting, site-specific pattern in the bird's brain, but the process of retrieving these memories remains unclear.
The Episodic Memory of Chickadees
The black-capped chickadees' ability to remember where they have stored their food in various environments is an impressive feat of memory. These birds can hide food in dozens of places and then locate them later, a process that requires the formation of episodic memory. Episodic memory involves recalling specific experiences from the past, and black-capped chickadees are "champions of episodic memory." Unlike humans who experience events repeatedly, birds have to form a memory instantly while caching food.
Chickadees have to remember not only the location of cached food but also other features of each hiding place, such as nearby trees, rocks, or other landmarks. They often have only moments to memorize all that information before moving on, and individual birds are known to cache up to 5,000 food items per day.
The Role of Hippocampus in Chickadees' Memory
Chickadees rely on the brain's hippocampus to form episodic memories, and the hippocampus is significantly larger in food-caching birds than in birds of similar size that do not cache food. Dmitriy Aronov and his team at the Zuckerman Mind Brain Behavior Institute at Columbia University aimed to investigate how neural activity represents the formation and retrieval of episodic memories in black-capped chickadees.
Designing a Lab Study
To study food caching in a laboratory setting, Aronov and his team designed a caching arena "optimized for chickadee ergonomics." The arenas included crevices covered by opaque flaps that the chickadees could open with their toes or beaks and cache food in. The chickadees did not require any special training to cache food in the arena; they naturally explored crevices and cache surplus food inside.
A microdrive attached to the birds' tiny heads and connected to a cable allowed live monitoring of their brain activity down to the scale of individual neurons. Scientists could see exactly where birds were caching seeds through a transparent floor and a camera aimed at the arena from below.
Profound Effect of Caching on Hippocampal Activity
Aronov and his team discovered that "the act of caching has a profound effect on hippocampal activity," with some neurons becoming more active during caching and others being suppressed. About 35% percent of neurons that are active during caching are consistently either enhanced or suppressed during caching—regardless of which site a bird is visiting. But the remaining 65% of variance is site-specific, and every cache is represented by a unique pattern of this excess activity in the hippocampus, a pattern that holds true even when two sites are just five centimeters apart—close enough for a bird to reach from one to another.
Retrieving Memory Patterns
Aronov's study revealed that every moment of caching creates a new, lasting, and site-specific pattern in a bird's brain. Chickadees can store thousands of moments across thousands of locations and retrieve those memories whenever they need extra food. However, it remains unclear how the retrieval process of the chickadees' episodic memories works. While Aronov's study showed that chickadees can reactivate site-specific brain activity patterns when they see one of their caches, it is still not clear how the birds actually remember which cache to visit for which specific item of food.
One possibility is that the birds use cues in their environment, such as landmarks or smells, to help them remember where they cached each item. Another theory is that the patterns of neural activity in the hippocampus serve as a sort of mental map, allowing the birds to navigate their environment and retrieve specific memories.
Regardless of how the retrieval process works, the fact that chickadees are able to store and retrieve thousands of site-specific memories is truly remarkable. This ability to create and maintain complex episodic memories sets them apart from other animals, including some primates.
Implications for neuroscience research
Studying the brains of chickadees has important implications for neuroscience research. As Aronov noted, understanding the neural mechanisms underlying the birds' remarkable memory abilities could lead to new insights into how memory works in humans and other animals.
Moreover, the fact that chickadees are able to form and retrieve site-specific memories in the wild makes them an ideal model organism for studying how memory works in naturalistic environments. By developing methods for studying chickadee memory in the field, researchers may be able to gain a more complete understanding of how episodic memory functions in real-world contexts.
In addition to shedding light on the workings of the brain, research on chickadee memory could have practical applications in fields like conservation biology. For example, understanding how birds remember where they've hidden food could help scientists develop more effective methods for tracking and monitoring bird populations in the wild.