Ornithologist Stefano Avilla froze in the middle of a muddy forest trail in October 2025, deep within the Brazilian Amazon close to Manaus. The morning mist had been broken by a single flute-like cry, the musician wren’s song. Jared Wolfe, a wildlife ecologist who works with him, silently nodded. They both knew the significance of that moment. It had become strange to hear it at all.
Deforestation is not the subject of this story. This area is free of encroaching farms, burn scars, and chainsaws. There is no damage to the canopy. The trees are quite old. These forests should be in good condition by all visible standards. Nevertheless, one species at a time, the birds are becoming silent.
For more than ten years, scientists keeping an eye on bird populations throughout the tropical Americas have seen this trend intensify. About twelve years ago, ornithologist Bette Loiselle and her partner John Blake noticed a problem at Ecuador’s Yasuní Biosphere Reserve: fewer birds were singing at sunrise in forests that didn’t appear to be having any problems.
Fourteen years of data followed, confirming their discomfort. Between 2001 and 2014, the number of birds captured in fine mesh nets decreased by 40%. A 50% decrease was revealed by systematic surveys. The most severely affected species were those that consume insects close to the forest floor, such as the black-tailed leaftosser and the rufous-capped antthrush. The losses persisted even after the anomalous weather patterns returned to normal.
“The first few years doing this I could stand in one place and in 15 minutes have 20 or 30 species,” Blake has said. “Now I can stand there and get two or three.”
The results were even more striking at Soberanía National Park in Panama, further north. Prior to 2020, the abundance of 70% of the 57 year-round resident species was lower than it was in the late 1970s. Nine had almost vanished.
Researchers discovered that birds from all 77 monitored species in Brazil, close to Manaus, weighed noticeably less in recent years than in the 1980s. The birds’ smaller bodies and thinner frames suggested that they were having trouble finding enough food. Among the species whose numbers had decreased by at least 50% was the musician wren.
It’s possible that numerous distinct pressures are stacked on top of one another in ways that are still difficult to decipher. However, one element consistently appears in the data: heat. According to a study published in the journal Nature Ecology and Evolution, extreme heat events decreased tropical bird populations by 25 to 38 percent between 1950 and 2020. Compared to forty years ago, there are now about ten times as many dangerously hot days in tropical regions. These are not the gradual, averaged-out temperature increases that climate models talk about in abstract terms. These are sharp increases in temperature, and they seem to be the main cause.
Tropical species evolved in a remarkably stable temperature range, in contrast to birds in temperate climates. They lack the genetic resources necessary for rapid adaptation. Their ranges are limited, their populations are frequently small, and their evolutionary flexibility is constrained. Some pant, some become disoriented, and some fall when the heat exceeds what their bodies can withstand. Reproduction is suppressed and organ damage results from repeated exposure. Sweating is absent. There’s no simple way out.
The fact that these are protected forests makes this especially challenging to comprehend. It has long been believed that wildlife within a habitat should be safe if it is protected from direct human intrusion, such as logging, burning, and fragmentation. That presumption now appears to be shaky. The effects of climate change extend beyond park boundaries. No matter where the birds go, the heat follows.
Researchers feel that the scientific community is still catching up to something that forest visitors have been observing for a long time. People who work long hours in the field have reported anecdotally that forests feel emptier, feeding flocks are getting smaller, and species diversity has decreased. What those observers suspected is now supported by the data. The declines seem to have accelerated between 2010 and 2012, when forests became noticeably hotter, drier, and less humid—conditions that have an impact on insect populations and, consequently, the birds that depend on them.
Sixty years ago, a completely different threat prompted Rachel Carson to warn about a silent spring. What’s happening now is slower and less obvious; it’s not caused by pesticides, but rather by temperature, drought, and the silent metabolic stress of animals that are living just a little bit outside of their natural habitat. There are still forests. What’s going on inside of them is the issue.
