Humans use recursion in language when we embed one clause within another to form a complex sentence, writes Scientific American's Diana Kwon. For example, if a human says, "The ball the bat hit flew," they've nested the clause "the bat hit" inside of "the ball flew." Scientists have long wondered whether understanding these patterns is unique to humans. "There's always been interest in whether or not nonhuman animals can also grasp recursive sequences," Diana Liao, the study's lead author who studies bird cognition at the University of Tübingen in Germany, tells Scientific American. In the early 2000s, linguists hypothesized that human language is the only form of animal communication that uses recursion, according to the Wall Street Journal's Dominique Mosbergen.