According to a recent scientific study, crows possess a remarkable cognitive ability once thought to be uniquely human: recursion — a core element of language complexity. This finding sheds new light on the intelligence of crows and challenges long-standing assumptions about the cognitive limits of non-human animals.

![Image credit: Ian Sane]

In the early 2000s, renowned linguist Noam Chomsky and his peers proposed that recursion was a fundamental and exclusive feature of human language. Recursion refers to the ability to embed structures within structures, allowing for infinitely complex and nuanced expressions. It was believed to be the defining trait that set human language apart from all other forms of animal communication.

However, this idea has gradually been called into question. A 2020 study demonstrated that rhesus monkeys are capable of understanding recursive patterns, with performance comparable to that of human children aged three to four — although the monkeys needed some additional training to reach this level. Now, a new study shows that crows, too, can grasp recursion, and perhaps even more efficiently than monkeys.

What Is Recursion, Exactly?

To understand the significance of this discovery, it helps to look at an example. Take the sentence: “The rat the cat chased ran.” It’s a bit tricky at first glance, but adult humans instinctively understand that “the rat ran” and “the cat chased (the rat).” This pairing of related elements — even when they’re embedded within a larger structure — is a hallmark of recursive thinking.

In simpler terms, recursion is the mental ability to recognize that one meaningful structure can be nested inside another. This ability allows humans to generate and comprehend sentences of virtually limitless complexity. It’s a foundational component of syntax and symbolic thinking — and now, apparently, not exclusive to humans or even primates.

Crows Rise to the Challenge

Researchers from the University of Tübingen in Germany sought to explore whether crows, members of the corvid family known for their intelligence, could understand recursive patterns as well. Using the same methodology as an earlier study conducted at the University of Wisconsin with rhesus monkeys, the scientists designed a test based on symbol sequences.

In the test, the crows were shown sequences made up of symbols like parentheses and brackets — for example, "<( )>". The task was to identify matching pairs of symbols, including those that were embedded within larger sequences. After mastering simpler sequences, the crows were challenged with increasingly complex symbol strings to assess whether they could continue identifying the nested, or recursive, structures.

Surprisingly, the crows succeeded in identifying the correct pairs in about 40% of the trials — a success rate that matched that of the rhesus monkeys. However, unlike the monkeys, the crows achieved this performance without the extensive training their primate counterparts required. This suggests that crows may have a more natural or intuitive grasp of recursive structure, at least under these experimental conditions.

![Are crows capable of understanding language? Image credit: h.koppdelaney]

Implications for Animal Cognition and Language

These findings are significant not only because they demonstrate recursion in birds but also because they broaden our understanding of animal intelligence across species. If crows — who are not primates — can understand recursion, then this cognitive skill may have evolved independently in different branches of the animal kingdom. It also opens the door to the possibility that other intelligent animals, such as dolphins, elephants, or even octopuses, might be capable of similar feats.

The results challenge the long-held notion that complex symbolic thought and hierarchical understanding are exclusive to humans or even to primates. They also reinforce what many scientists have suspected for years: that crows, along with other corvids, are among the most intelligent animals on Earth.

While the crows' ability to understand recursion doesn’t mean they possess language in the human sense, it does indicate that they are capable of recognizing and processing abstract patterns — a foundational step toward more complex communication systems.