Black-capped chickadees have a vocabulary of 50 different vocalizations, including their namesake alarm call: chicka-dee-dee-dee. The longer the call lasts, the more dangerous the predator they’ve spotted.
Another small songbird, the red-breasted nuthatch, recognizes this alert, but a new study in Nature Communications shows that they don’t take the chickadees’ calls at face value. When a nuthatch hears a chickadee cry out, it will start to sing its own alarm call. But until they hear another nuthatch reply to confirm the predator sighting, they don't take wing.
Erick Greene, a University of Montana ecologist and co-author of the new study, explains it's kind of 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," he tells National Geographic’s Brian Gutierrez.
In 2007, Greene first found that nuthatches were eavesdropping on chickadee conversations by hiding speakers at the bases of trees and playing chickadee alarm calls, per the paper published in the Proceedings of the National Academy of Sciences.
“Chickadees and nuthatches are found together throughout most of the country. And in fact, they actually form winter flocks together,” biologist Chris Templeton of Pacific University, who coauthored both papers, told NPR’s Renee Montagne and Steve Inskeep in 2007. “They're both small, little tiny birds and so they share the vast majority of their predators.”
When chickadees spot predators, they sound the alarm for any animal in earshot. Then, other birds, like nuthatches, go on alert. When the nuthatches spot the suspicious raptor, they’ll put out a “mobbing” call—a rapid series of chirps—and start to harass the predator as a group.
In the new study, Templeton, Greene, and ornithologist Nora Carlson observed nuthatch reactions to different chickadee alarm calls, and compared those to nuthatch reactions to the calls of predators—the very dangerous pygmy owl, and the less dangerous great horned owl, and two control birds, the house sparrow and the Townsend’s solitaire. They found that nuthatches sing out their mobbing calls when they hear predators directly, but not when they hear chickadee alarms alone.
It’s not yet known why nuthatches don’t fully trust their chickadee companions, Eastern Kentucky University ornithologist Gary Ritchison, who was not involved in the study, tells National Geographic.
“One possible explanation 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,” he says.
A Goldilocks-precision response to a threat, without over- or under-reacting, is important for the small birds’ survival, particularly at times when resources are scarce, the researchers write in Nature Communications. If nuthatches sing out their mobbing calls for a false alarm, they would waste precious energy; But ignoring the calls might mean missing a life-saving warning.
As Greene tells National Geographic, natural selection seems to have favored nuthatches that “retweet” cautiously.