You might go for a walk in the forest to disconnect from work and calm your nerves after a busy week. The chirping and calls of birds in the canopy above might be exactly what allows you to relax.
But what sounds soothing to humans may signal danger to other animals - and trigger fear across the forest.
In our research , published today in Current Biology, we show that when some animals spot a predator they issue a warning cry that is 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.
Birds and monkeys
During an expedition to a remote area of the Peruvian Amazon, working with a falconer, we used trained raptors to trigger warning calls from birds and primates. We recorded the calls then played them back into the forest and monitored how the community responded.
We already knew that birds sometimes repeat the warnings of others - occasionally even those of different species, or of primates. What we wanted to know was how widespread this behaviour is across the animal community.
We 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 neighbours throughout the forest.
Sounds and silence
Alarm calls from species living in the forest understorey were far less likely to spread and be propagated by other birds or primates.
However, even when these alarm calls were not repeated, they changed the forest's soundscape. Small canopy birds almost completely stopped singing after hearing a predator alert. At the same time, animals in lower forest layers often continued to make sounds despite the perceived threat.
Together, these findings suggest that the Amazonian canopy is not only the rainforest's most mysterious layer - largely unexplored and home to much of its biodiversity - but also functions as an information highway, like a fibre-optic network through which animals rapidly share signals of danger.
A new layer of the 'internet of the forest'
In the past decade, the idea of an "internet of the forest" has become popular through the concept of the "wood wide web", where plants exchange resources and information via root systems and fungal networks. Our work points to another communication system, one operating high above the ground.
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.
The vocal activity of birds is usually associated with finding mates and defending territories. However, we now know that sometimes this activity, or lack of it, may represent pulses of a soundscape of fear.
Next time you walk through a rainforest, look up and listen to the birds. A sudden silence may mean a raptor is gliding somewhere above the canopy.
The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
