Crows are notoriously clever, and some of the smartest animals in the Natural World. Now, a new study suggests their grasp of a complex cognitive principle is better than that of macaques and young children.

Crows Understand Recursion

Researchers discovered crows can distinguish paired elements buried in larger sequences — a cognitive ability known as recursion.

“Recursion, the process of embedding structures within similar structures, is often considered a foundation of symbolic competence and a uniquely human capability. To understand its evolution, we can study the recursive aptitudes of nonhuman animals. We adopted the behavioral protocol of a recent study demonstrating that humans and nonhuman primates grasp recursion. We presented sequences of bracket pair stimuli (e.g., [ ] and { }) to crows who were instructed to peck at training lists. They were then tested on their ability to transfer center-embedded structure to never-before-seen pairings of brackets.” —Abstract of 2024 study in Science Advances

Consider the sentence: The cat the dog chased booked it down an alley.

Though the sentence is a grammatical nightmare, most adults understand the cat booked it down an alley and the dog chased the cat. This capacity to pair elements such as “cat” to “booked it” and “dog” to “chase” in the above sentence/sequence was once thought to be a uniquely human trait. But crows have the same ability.

This new research builds on previous work that tested recursive reasoning in macaques.

“One of the most distinguishing features of human communicative cognition may turn out to be not that human-specific after all,” said lead researcher/author Diana A. Liao, a postdoctoral candidate, who conducted the study at the University of Tübingen in Germany.

Recursion occurs outside of human language. Consider a musical phrase within a larger piece or a mathematical expression embedded within parentheses. In a 2020 study published in Science Advances, researchers proved humans can follow recursive patterns without a formal background in reading and mathematics, which they demonstrated by including isolated Amazonian tribe members.

In the same study, macaques were only slightly inferior to toddlers when it came to distinguishing paired elements, such as opened and closed brackets, from a plethora of symbols.

The new study published November 2, 2024, in Science Advances builds on this work to extend the findings beyond primates.

“The study is well-designed and executed, and the results are clear and compelling,” said Stephen Ferrigno, lead author of the 2020 findings and an assistant professor in the psychology department at the University of Wisconsin-Madison.

Ferrigno was not involved in the new study.

Liao and her colleagues taught crows to identify the symbols { }, [ ], and < > by rewarding them with treats when they pecked the correct order of center-embedded recursive sequences. It took the crows about a week to learn to peck the symbols, after which the crows sat for their final exam, which included strings of similar symbols they had not seen before. Humans and macaques faced with the same test usually understood if {( )} is correct then [{ }] is also correct.

Not only did the crows do as well as children, they outperformed the macaques. This result suggests the ability to identify recursive sequences may have evolved for purposes outside of the language domain.

“The crows continued to produce recursive sequences after extending to longer and thus deeper embeddings. These results demonstrate that recursive capabilities are not limited to the primate genealogy and may have occurred separately from or before human symbolic competence in different animal taxa.” —Abstract of 2024 study in Science Advances

Recursive logic might also be a key component of communication for crows.

“If corvid songbirds can understand and produce recursive structures, they may also use it for vocal communication and managing their intricate social relationships.” ~Diana Liao

When I read that quote, I was immediately reminded of the male wood thrush, who sings ee-oh-lay, ee-oh-lay in the middle of a three-part sequence. Males try to out-sing each other in a contest of sorts, where they compete for who sang the sweetest song. Or they sing the same melody over and over to win the title of who sang it best.

Could these tiny forest songbirds understand the concept of recursion?

After all, the males also sing “internal duets.” In the final trilling phrase of their three-part song, they sing pairs of notes simultaneously, one in each branch of their y-shaped syrinx (voice box). The two parts harmonize to produce a haunting, ventriloquially sound.

Amazing, right?

Here’s the gut-wrenching part: Humans are pushing the wood thrush to the brink of extinction. Right now, they are on the Yellow Watch List for birds most at risk for extinction without significant conservation efforts to reverse habitat loss and forest fragmentation. The wood thrush is one of the most prominent examples of declining forest songbirds in North America.

That notion tears me up inside. Imagine a nature walk without birdsong?

The male wood thrush’s magical, flute-like song echoes through the forest. They sing several variants with eight to ten loud, clear notes. In combination with the introductory phrase’s low, soft notes and six to twelve variants of the final higher-pitched complex trill, a male wood thrush can easily sing over fifty unique songs. Though the female wood thrush can also sing, it’s most often the males we hear due to their competitive nature.

Fun Fact

All birds veered off on their own evolutionary path before dinosaurs reached extinction. Scientists agree birds are dinosaurs (!!) because they directly descended from theropods, the common ancestor of all dinosaurs. Other than our avian friends, there is no scientific evidence of non-avian dinosaurs — carnivorous, walked on two legs — still in existence.

Final Thought on Recursion

The new research published in November 2024, made me wonder how many other species understand recursion. I bet whales, Orca, and dolphin do.

Science has only begun to scratch the surface of animal intelligence.

What do you think of the new study?

A crow’s intelligence rarely surprises me anymore — still fascinates me, though — but their recursion ability blew my mind… as did their ability to understand the concept of zero.

Crows Understand Zero

Think about what zero means in a number system. The notion of, say, 5 x 0 = 5, or 0 + 10 = 10, didn’t develop in humans till around the fifth century A.D. The concept of “none,” or the absence of any quantity, likely emerged earlier, but it differs from using zero as a distinct quantity.

“If you ask mathematicians, most of them will probably tell you that the discovery of zero was a mind-blowing achievement,” said Andreas Nieder, a professor of animal physiology at the Institute of Neurobiology, University of Tübingen in Germany. “The special thing about zero is that it doesn’t fit into a routine of counting real objects…”

For example, we can count three apples or peaches in a basket—one, two, three—but when the basket is empty, there is no fruit to count. Zero represents the emptiness, the absence of fruit in the basket.

“That obviously requires very abstract thinking… thinking that is detached from empirical reality.”

Different species of animals can determine the countable number of objects in a set. However, despite its importance for human number theory, the empty set “zero” had remained largely unexplored. Understanding the concept of zero as a meaningful numerical category demands high-level intelligence. Still, cognitively advanced animals, like macaques and honeybees, possess a primitive non-symbolic notion of zero.

How Did Crows Do?

In the study published in The Journal of Neuroscience, when Nieder and colleagues peered into the brains of smarty-pants crows, they discovered their nerve cells, or neurons, encode “zero” as they do other numbers. By inserting a thin wire into the crows’ brain, activity patterns supported their findings that 0 came before 1 in a number sequence. In other words, crows already valued zero in their “mental number line” and grasped the empty set as a null numerical quantity that’s mentally represented next to number one.

I’ve written about the counting ability of crows before.

In fact, I included this amazing ability in Unnatural Mayhem, when Poe and his brethren found themselves in a dangerous position. If Poe and the local murder couldn’t count, they might not escape thirty armed crow hunters.

How did Poe use this critical skill? Guess you’ll have to read the book to find out. 😉

Wishing you and yours a joyous holiday season!