But sometimes we use syntax to impart complex combinations of ideas. "Careful, it's dangerous" is a phrase that has meaning, and so is "come toward me." When those two phrases are combined, they have a different meaning than they do on their own: They're directing the receiver to act in a different way than either phrase would independently.

Until now, only humans seemed to use syntax this way. But a study published Tuesday in Nature Communications suggests that the Japanese great tit — a bird closely related to the North American chickadee — uses grammatical rules like these in its calls.

All language, human and otherwise, revolves around turning meaningless sounds into something more. It's widely accepted that many non-human animals use what's called referential communication — specific sounds mean specific things to the receiver. Beyond that, there are two kinds of syntax that make speech more complicated, but also more useful: phonological and compositional. Humans have both, and until this new study, non-human animals had only been shown to have the former.

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Phonological syntax turns sounds that individually have no meaning into ones with meaning. Suffixes and prefixes are a good example in human language, and other animals use strings of different notes that are never used individually. The Campbell's monkey adds an "oooh" sound to the end of its vocalizations to increase the intensity of the message, and this sound is never used on its own — so that's another example. Another study found that some birds won't respond to calls unless the notes involved are made in the right order.

"In the course of 10 years of field research, I noticed that the Japanese great tit has a wide variety of call types and uses many different calls in different contexts," lead author Toshitaka Suzuki of the Graduate University for Advanced Studies told The Post. In a previous study, Suzuki showed that the birds used these complex calls as "words" that conveyed different meanings. He wondered if they might also string those words together to form compound messages.

It turns out they do — and the order of the message might matter in the same way it does when humans speak to one another.

Suzuki and his colleagues found that a call referred to as the "ABC" call — a string of notes used to signal other birds to scan for predators — was often followed by the "D" call, which told other birds to approach. When the ABC-D call is made, birds were seen to conduct both behaviors: They flew toward the speaker but scanned for predators first.

"The really critical part of the study came out of the review process," co-author David Wheatcroft, a postdoctoral researcher at Uppsala University, told The Post. When scientific studies are submitted for publication, uninvolved scientists in the same field have to screen the research and declare it sound. In this case, both reviewers asked how the study authors could be sure that the particular combination of ABC-D was imparting a specific, compound meaning.

"Maybe they just hear ABC and D in close proximity and do both behaviors," Wheatcroft explained. "So we said okay, we'll do another experiment. Toshitaka reversed the call, and he played D-ABC for the birds. And they didn't respond, or at least not as strongly or consistently as they did to ABC-D."

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So instead of just combining calls and producing two behaviors, there seems to be some kind of rule to prompt the compound behavior: approaching, but staying on high alert for predators lurking nearby.

The researchers aren't sure what the rule is based on. Perhaps, since the predator-related piece of the call is more important, the birds have created this "rule" over time. Maybe syntax developed because birds that didn't warn of danger before asking for a buddy to come over were less likely to survive. And no one wants to blunder into a predator's line of sight on the way to a friend's place

The next step is to find out which other birds use rules like these. The Japanese great tit has close relatives in Europe, and the North American chickadee is a close cousin as well. They all have similarly complex calls — the chickadee gets its name for a "dee" sound that serves the same purpose as the "D" discussed in the study — and it's possible they all use syntax.

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Even if the birds evolved this love of grammar way back in the days of a common ancestor, their specific rules could still be different.

"Do they have the same rule? Maybe the ordering is switched in North America. Maybe the syntactical rule is random," Wheatcroft said.

He expects scientists to find evidence of these grammatical rules in other birds — and, he hopes, across the animal kingdom.

"We hope people start looking for it and find it everywhere," he said. "Because then we can start answering the question of how and why syntax evolved. For now, we don't have any close relatives that we know who use syntax. And it's a big question. Why not just convey a new meaning by creating a new word? Why does order matter? We hope that in the future, this research will help give us insight into why syntax evolved in humans."