These birds 'retweet' alarm calls—but are careful about spreading rumors
Red-breasted nuthatches don't trust everything they hear, suggesting evolution favors cautious behavior, a new study says.
If you live in North America, you might have enjoyed the bright songs of black-capped chickadees or red-breasted nuthatches on your street. But you might not have known that those songs have lyrics.
“You might call them words,” says Erick Greene, an ecologist at the University of Montana, “but linguists might get upset about that.”
In fact, the chickadee has a vocabulary of around 50 distinct sounds that communicate a few essential phrases, like “danger!” “feed me!” or “I’m single!”
Greene and his colleagues have previously found that nuthatches eavesdrop on these chickadee warning signals and “retweet” them to their neighbors—like real-life Twitter, Greene jokes.
But their new research shows that like any good reporter, the nuthatch checks out its facts—the birds will repeat the general chickadee alarm call, but they don’t vocalize more specific information about the predator until they can verify it, according to the study, published January 27 in the journal Nature Communications. (Listen to what could be the loudest bird on Earth.)
Such research is important because it builds on ecologists’ growing understanding of communication networks between animals in the same environment. What's more, listening in on birds’ specific alarm calls can give scientists a rough estimate of how many predators are living in a particular area—an effective, non-invasive way of measuring the health of the ecosystem.
Though each bird species has its own “language,” certain calls cut across cultural and geographic borders. For example, a seet is the universal danger call made by birds and small mammals.
Chickadees, it turns out, chirp about predators a lot. In fact, their name comes from the warning call they make when they see a suspicious character lurking around the neighborhood: Chickadee-dee-dee-dee-dee. The more “dees,” the more dangerous the predator.
The chickadee call is a little different from a seet. Instead of “Run for your life!” it’s more like, “Sing for your life!” in the sense the vocalization riles up other birds to jump out and harass a potential predator, like an owl or a hawk, in a behavior called mobbing. (Read why birds matter, and are worth protecting.)
The sounds are “really obnoxious,” Greene says, like “dragging your fingernails on an old-school chalkboard.”
For this experiment, Greene and his colleagues placed speakers in 60 locations near Missoula, Montana, and in three locations near Mazama, Washington, all of them near known populations of wild nuthatches. The team then played recorded sounds of the very dangerous pygmy owl, the less dangerous great horned owl, and a third control bird that does not pose any threat to the nuthatch.
In the second stage of the experiment, scientists played warning calls of chickadees that had seen the same predators. Then they recorded the sounds that the nuthatches made to see if the birds responded to hearing about a predator secondhand in the same way that they would if they heard the predator directly.
Why check it out?
Greene and colleagues found that when the nuthatches heard the predator calls directly, they would put out their own mobbing call—a series of rapid, single-syllable chirps—to their own kind that communicated the size of the threat. Bigger threats, such as the pygmy owl, produced shorter, higher-pitched calls.
When nuthatches heard alarm calls from the chickadees, the birds would repeat the call with a vague, general warning call—regardless of the threat level.
It’s unclear why the nuthatches don’t repeat the same level of alarm that they hear from the chickadees, but the researchers speculate that the information from the chickadees could be unreliable, so repeating the call—but with less certainty—is a good survival strategy.
It's like the nuthatches are saying: "We're on high alert and we got it from the chickadees that there's something out there, but we haven't verified it," Greene says.
Gary Ritchison, an ornithologist at Eastern Kentucky University, agrees it’s unknown why the nuthatches don’t trust the chickadees enough to repeat their level of alarm. (Read why small birds have sweeter songs.)
“One possible explanation,” says Ritchison, who wasn’t involved in the study, “is the nuthatches “know where the chickadee is, but that doesn't provide much information in terms of where the possible predator might be located.”
There are many reasons why a secondary source of information is less reliable than firsthand knowledge, so it’s possible that natural selection has favored nuthatches that are cautious with repeating rumors—even if they are incredibly alarming, the study says.
If only humans were so discerning.