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Zebra finches have a more complex way of communicating than previously thought. A new study in the journal Science found that the sociable songbird does not just recognize and organize different calls based on how they sound but also on what they mean. This suggests they have a deeper understanding of their own language.

Scientists already knew that zebra finches produce around 11 different types of calls (ethogram-based, or Etho calls) to communicate things like hunger and danger. What they didn't know was whether the birds perceived these calls as distinct categories or just a jumble of noise.

More than just sounds

To figure out how the birds understand their calls, a research team led by Julie E. Ellis of the Helen Wills Neuroscience Institute at the University of California, Berkeley created a kind of game. They trained 12 adult finches (six males and six females) to peck a key to get a treat, but only when they heard a specific "target" call known as the Distance call.

If they heard a different non-target call (the Tet call) and pecked the key, the sound would stop, and they wouldn't receive a reward. The finches had to listen to many different versions of these calls, even from birds they had never heard before, so they couldn't just memorize the sounds.

The research team then played sounds that were a mix of the two, gradually transitioning from one to the other. For all the sounds that were mostly a Tet call, the birds consistently refused to press the key. But as soon as the sound crossed a certain point and became more like a Distance call, the birds' behavior suddenly switched. They immediately start pressing the key to get a reward. In other words, they acted as if there was a clear distinction between the two calls.

This initial test demonstrated that the birds could categorize their calls. But did they understand what they meant? To find the answer, the researchers looked at mistakes the birds made when trying to identify all 11 calls. They found that when they confused calls, they were more likely to confuse two calls with a similar purpose. The scientists dubbed this the "semantic magnetic effect," because calls with a similar purpose were being pulled together in a bird's mind.

"Zebra finches confused Etho call types that have a similar meaning at a higher rate than would be expected given the acoustic similarity of these call types. Postulating that hearing calls elicits mental representations of call meaning could explain this semantic magnet effect," wrote the researchers in their paper. "Zebra finches organize calls by meaning, not just sound."

Future work may look at the brain mechanisms that allow finches to organize their calls by meaning. Studying other species, such as dolphins or bees, may also teach us more about how complex communication systems work in the animal kingdom.