By faithfully copying the most popular songs, swamp sparrows (Melospiza georgiana) create time-honored song traditions that can be just as long-lasting as human traditions, according to a new study, published in the journal Nature Communications. In fact, song traditions in swamp sparrows often last hundreds of years, with some songs going back further than that.
The swamp sparrow is a medium-sized sparrow related to the song sparrow (Melospiza melodia).
The slow trill of this species can be heard in marshes and wetlands across eastern and central North America.
The bird attracts mates and defends its territory with songs built from two- to five-note snippets, repeated over and over.
A few decades ago, ornithologists observed that swamp sparrows living in different places sing slightly different songs: birds in New York might tend to sing in three-note repeats while their counterparts in Minnesota favor four, or combine the same basic notes in a different order; young birds learn the local customs in the first weeks of life by imitating their elders.
But while similar cultural traditions — shared behaviors that are learned from others and passed from one generation to the next — have been observed in all sorts of animals, the thinking has been that human traditions are more likely to last.
To test the idea, Dr. Robert Lachlan from Queen Mary University of London and colleagues recorded the songs of 615 male swamp sparrows in six populations across New York, Pennsylvania, Michigan and Wisconsin.
Using computer software to measure and analyze each song, the researchers identified 160 song types across the species’ range.
Each male has only a handful of songs in his repertoire. To figure out how young birds choose which songs to learn, the scientists developed a mathematical model that simulates how each new song type spreads within groups over time.
Each run of the model represented 5,000 years, at the end of which they measured the song types in each group of birds.
With their model they also compared various song-learning strategies. For example, young birds might prefer to imitate one particular adult, such as their dad or a male with a good territory. Alternately, they might pick certain songs because they find them inherently more attractive, regardless of who sings them.
When they looked at how well their simulations fit the real data, the study authors found that young birds don’t just randomly pick any song they hear and imitate that. Instead, they copy the crowd, mimicking the most popular songs more often than one would expect by chance. Unique or rare songs that go against the mainstream rarely get a peep.
“It’s called a ‘conformist bias’,” Dr. Lachlan noted.
“What’s more, swamp sparrows learn their songs with amazing fidelity, correctly matching the songs they attempt to imitate more than 98% of the time,” the researchers said.
“There’s an evolutionary benefit to fitting in. Previous studies show that females prefer typical tunes over outliers.”
The end result, their models show, is that local song customs in swamp sparrows are far from fleeting trends, quickly going out of fashion and never to be uttered again.
Instead, they are handed down from one swamp sparrow generation to the next, with song types often persisting for 500 years or more.
“According to the models, some of the songs could go back as far as the Vikings,” Dr. Lachlan said.
Robert F. Lachlan et al. 2018. Cultural conformity generates extremely stable traditions in bird song. Nature Communications 9, article number: 2417; doi: 10.1038/s41467-018-04728-1