Cultural conformity generates extremely stable traditions in bird song

Robert F. Lachlan, Oliver Ratmann, Stephen Nowicki•View original
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Think about the first song you ever learned by heart. You didn't invent it from scratch. You listened, practiced, and copied what everyone around you was singing. Swamp sparrows do the same thing. Their songs are learned, not hardwired, and that makes them a window into culture in the wild: how traditions start, spread, and last. The big question is simple and deep. How faithful does copying have to be, and does going with the crowd turn individual learning into something that endures? Robert Lachlan, Oliver Ratmann, and Stephen Nowicki went big to answer that. They recorded six hundred fifteen adult male swamp sparrows across six populations in the eastern United States and compared the shapes of their syllables — the little building blocks of song. To do that, they used software that stretches and aligns sounds so you can judge similarity, a bit like matching two melodies by bending time so the peaks and valleys line up. Out of this, they identified one hundred sixty distinct syllable types. Birds in the same population shared many of these, birds farther apart shared fewer, and the popularity of syllables had a long-tailed look — a few hits and lots of obscurities. That pattern hints at culture diverging across space and a crowd that plays favorites. Then they asked, what learning rules could produce what we see? Instead of writing down equations, they built models and let them run. They simulated populations over five thousand years and used a technique called Approximate Bayesian Computation with Population Monte Carlo. Think of it as spinning up thousands of virtual worlds and keeping the ones whose summary numbers match the real data. To keep the comparison manageable, they boiled many song statistics down to a handful and tightened the match step by step. Here's what falls out. Copying is very accurate. The estimated error rate — how often a learner's version becomes a genuinely new syllable — is about one point eighty-five percent. And copying isn't neutral. There's a clear conformist bias: a parameter they call alpha comes out around one point three one six, meaning learners are measurably more likely to adopt syllables that are already common. In plain terms, young birds don't just copy; they copy the hits. What could cause that without fancy cognition? The team points to a simple, two-phase routine. Juveniles overproduce, trying out several syllables, and later prune the set, keeping the ones they've heard most. In their fitted model, the birds sample roughly three tutors early on, around three again later, and give extra weight — about one and a half times — to syllables they've encountered before. Try many, keep the popular. That's enough to tilt the culture toward the common. Put those pieces together and traditions suddenly have legs. In simulations matched to the field data, the average oldest syllable type is about one thousand five hundred thirty-seven years old. And it's not just a few Methuselahs. Roughly eight point six percent of syllable types — accounting for about twenty-six point five percent of all sung syllables — are older than five hundred years. For a song you learn by ear, that's astonishing longevity. The real world echoes that stability. At one site with recordings from the 1970s and again in two thousand nine, all but two of the nineteen syllable types sung by multiple males in two thousand nine were already there decades earlier, and the most common type then was still the king later. Within populations, sharing stays high. Between populations, it drifts, just as you'd expect for cultures separated by distance. So what should you carry with you on your walk? Three things. First, social learning in these birds is precise — errors are rare. Second, youngsters favor the crowd — conformity is real, and it matters. Third, that simple combo can sustain bird song traditions for centuries, rivaling the durability of some human customs. As Lachlan and colleagues show, you don't need complex minds to get rich culture. You need faithful copies, a nudge toward the popular, and time.

Think about the first song you ever learned by heart. You didn't invent it from scratch. You listened, practiced, and copied what everyone around you was singing.

Swamp sparrows do the same thing. Their songs are learned, not hardwired, and that makes them a window into culture in the wild: how traditions start, spread, and last. The big question is simple and deep.

How faithful does copying have to be, and does going with the crowd turn individual learning into something that endures?

Robert Lachlan, Oliver Ratmann, and Stephen Nowicki went big to answer that. They recorded six hundred fifteen adult male swamp sparrows across six populations in the eastern United States and compared the shapes of their syllables — the little building blocks of song. To do that, they used software that stretches and aligns sounds so you can judge similarity, a bit like matching two melodies by bending time so the peaks and valleys line up.

Out of this, they identified one hundred sixty distinct syllable types. Birds in the same population shared many of these, birds farther apart shared fewer, and the popularity of syllables had a long-tailed look — a few hits and lots of obscurities. That pattern hints at culture diverging across space and a crowd that plays favorites.

Then they asked, what learning rules could produce what we see? Instead of writing down equations, they built models and let them run. They simulated populations over five thousand years and used a technique called Approximate Bayesian Computation with Population Monte Carlo.

Think of it as spinning up thousands of virtual worlds and keeping the ones whose summary numbers match the real data. To keep the comparison manageable, they boiled many song statistics down to a handful and tightened the match step by step.

Here's what falls out. Copying is very accurate. The estimated error rate — how often a learner's version becomes a genuinely new syllable — is about one point eighty-five percent.

And copying isn't neutral. There's a clear conformist bias: a parameter they call alpha comes out around one point three one six, meaning learners are measurably more likely to adopt syllables that are already common. In plain terms, young birds don't just copy; they copy the hits.

What could cause that without fancy cognition? The team points to a simple, two-phase routine. Juveniles overproduce, trying out several syllables, and later prune the set, keeping the ones they've heard most.

In their fitted model, the birds sample roughly three tutors early on, around three again later, and give extra weight — about one and a half times — to syllables they've encountered before. Try many, keep the popular. That's enough to tilt the culture toward the common.

Put those pieces together and traditions suddenly have legs. In simulations matched to the field data, the average oldest syllable type is about one thousand five hundred thirty-seven years old. And it's not just a few Methuselahs.

Roughly eight point six percent of syllable types — accounting for about twenty-six point five percent of all sung syllables — are older than five hundred years. For a song you learn by ear, that's astonishing longevity.

The real world echoes that stability. At one site with recordings from the 1970s and again in two thousand nine, all but two of the nineteen syllable types sung by multiple males in two thousand nine were already there decades earlier, and the most common type then was still the king later. Within populations, sharing stays high.

Between populations, it drifts, just as you'd expect for cultures separated by distance.

So what should you carry with you on your walk? Three things. First, social learning in these birds is precise — errors are rare.

Second, youngsters favor the crowd — conformity is real, and it matters. Third, that simple combo can sustain bird song traditions for centuries, rivaling the durability of some human customs. As Lachlan and colleagues show, you don't need complex minds to get rich culture. You need faithful copies, a nudge toward the popular, and time.