SANTA CRUZ, Calif. – During an expedition to a remote area of the Peruvian Amazon, working with a falconer, members of the Forest Fear Lab at the University of California, Santa Cruz, used trained raptors to trigger warning calls from birds and primates. The team recorded the calls, then played them back into the forest and monitored how the community responded.
The researchers already knew that birds sometimes repeat the warnings of others—occasionally even those of different species, like primates. What they wanted to know for this project was how widespread this behavior is across the animal community.
Their study, recently published in the journal Current Biology , found that when some animals detect a predator, they sound a warning cry that gets picked up by others and spread through the rainforest canopy. For a time, different species are linked into a shared information network, and parts of the forest briefly fall silent.
"Animals, regardless of how diverse the community is—whether it's birds or mammals—share predators. So they recognize each other's warnings," said study co-author Ari Martínez, assistant professor of ecology and evolutionary biology at UC Santa Cruz. "It behooves them to recognize that information about the predators and then propagate it to others."
Fear in the air
The densest part of the rainforest is the canopy, where most animals live and where sound travels quickly. Beneath the canopy is the understory, a quieter place where sunlight is sparse but birds, lizards, and insects are abundant.
Enter Martínez's Forest Fear Lab , which researches how animals share and use social information across natural and urban environments, and how these communication networks vary along environmental and evolutionary gradients—influencing community dynamics.
In this new study, the researchers say their findings underscore the importance of studying communication networks at the community level, opening new avenues for exploring how widespread and ecologically significant these patterns are—not only in tropical ecosystems, but also beyond.
"There is so much information in an alarm call, but the first note is really important," said Ettore Camerlenghi, the study's lead author and an associate research fellow at Deakin University in Melbourne. "Because if you need to escape from a predator, you cannot be there and wait to decide what to do. You just need to move—a millisecond might save your life."
Specifically, the team discovered that alarm calls produced by small bird species—those weighing less than 100 grams—were most often passed on. Other small birds living in the canopy were the most likely to relay the call, but other animals joined in, too.
Larger species, including capuchin and spider monkeys, sometimes responded as well. Two canopy species in particular, the black-fronted and the white-fronted nunbirds, stood out as especially likely to repeat and propagate the warnings of their neighbors throughout the forest.
"That's the beauty of vocal information: It's public information, and once it's there, it's for everybody who can get it," Camerlenghi explained. "It's especially important in such a dense environment like the rainforest."
'Internet of the forest'
In the past decade, phrases like the "internet of the forest" and "wood wide web" have gained popularity, referring to natural networks where plants exchange resources and information via root systems and fungal networks . This work points to another communication system—one operating high above the ground.
The vocal activity of birds is usually associated with finding mates and defending territories. However, this area of ecology research reveals that sometimes this activity, or lack of it, may represent pulses of a soundscape of fear.
"Suspended above our heads is a vast ecosystem where animals constantly listen to one another, forming an eavesdropping network that spreads critical information within seconds," Camerlenghi and Martínez wrote for the Conversation .
Researchers still don't fully understand all the information that alarm calls are communicating. But some speculate they could tell other birds where predators are located and more. Martínez, a National Geographic Explorer, said future studies could quantify the spread of information through acoustic technology and create maps of the sounds over space and time.
