Some animals share and pass down knowledge, creating cultural traditions, but one species of bird is really good at it.
The American swamp sparrow (Melospiza georgiana) copies the most popular songs in its community — some of which may date back to the time of the Vikings, according to a study published in Nature Communications.
"The longstanding stable traditions so characteristic of human behavior have often been ascribed to the high cognitive abilities of humans and our ancestors," study co-author and professor of biology at Duke Stephen Nowicki said in a statement from Duke. "But what we're showing is that a relatively simple set of rules that these songbirds are capable of following can achieve equally lasting traditions."
No. 1 song
To find exactly what the sparrows learned, researchers took song samples from 615 swamp sparrows from six densely populated areas across the northeastern United States between 2008 and 2009. The songs were then picked apart by an acoustic analysis program, providing the researchers with the songs' notes, or syllables, and the diversity between the songs.
When they're young, the sparrows learn dozens of syllables, but as they age, they tend to use the same three over and over again. So while some birds, a mere 2 percent, might create some new kinds of music, the rest of the birds stick with the traditions of their region, using their particular three notes to sing the same song that their ancestors likely sang.
That remaining 98 percent learn from their elders, like their daddy bird or a male with good territory. Sometimes, they just like the song, regardless of which other bird is singing it. So as new birds grow up in a territory, they learn the most popular songs and repeat them, often times perfectly.
It's a process called conformist bias. We take social cues from those around us, assuming that it's the correct decision, instead of relying on our judgment. (When in Rome and all that.) So because the swamp sparrows are hearing the same few songs over and over, they assume those are the correct songs, and whatever outliers there are aren't worth learning.
This method of song learning filters out new songs, of course, since they're less popular. Given the lack of variation then, it may not be surprising to learn that with the exception of two syllables, the most common syllables researchers recorded were also the most common syllables from recordings of the sparrows taken during the late 1970s.
To determine the evolution of songs, the researchers ran the notes through a statistical calculation model called the Approximate Bayesian Computation, which provides simulated data based on empirical data, in this case the songs recorded during the study's time period. The oldest and most popular syllable in each of the six observed populations would be 1,537 years old, and nearly 9 percent of syllable types were more than 500 years old. Older syllables were more common than newer syllables.
Of course, this is just a model, and as the researchers point out, it can't take into account things like "the temporary extinction and re-colonization of a marsh population due to changes in water level." So it's not direct evidence of the song's stability, but the models do "suggest that, under stable conditions, syllable type traditions can last for a very long period."
And those stable conditions are important to the success of these songs. Human intervention in nature by way of roads and cities, to say nothing of habitat loss, can turn a unified population of species into a divided one that rarely interacts. This kind of fragmentation can hinder the ability of songbirds to learn.
Still, that the birds are singing basically the same tunes since at least the late '70s is impressive.
"We're not saying that birds have anything akin to human culture," said Robert Lachlan, a lecturer in psychology at Queen Mary University of London and the study's lead author. "It shows that just those two ingredients — a preference for popular songs, and the ability to copy them — can get you quite a long way to having stable complex culture."