Photo by Andres Vasquez Noboa / Macaulay Library.
From the Spring 2025 issue of Living Bird magazine. Subscribe now.
How healthy are bird populations living deep in pristine tropical forests?
The conventional hope has been that, even while bird populations in the Northern Hemisphere are declining, species living in tropical forests may be insulated because they are less exposed to human activities. A series of recent studies has challenged this conventional wisdom, however, and suggested that populations may be declining even in apparently undisturbed forests. Now, a new study uses a unique long-term database to shed light on why that might be the case. It makes for unsettling reading.
The new paper by Michigan Tech ornithologist Jared Wolfe and colleagues in the U.S. and Brazil reports on the results of 27 years of painstaking research, observing and catching birds in the middle of the Brazilian Amazon, at the Biological Dynamics of Forest Fragments Project, coordinated by the Instituto Nacional de Pesquisas da Amazônia. The paper lays the cause of the population declines at the door of climate change, with the annual survival of birds in 24 out of 29 species studied being significantly lower when the dry season was harsh. The changes in survival are dramatic, especially given the changes in temperature and rainfall are relatively small. A less than 2°F increase in temperature corresponded with a 63% decrease in survival, while a less-than-half-inch decrease in rainfall lowered survival by 14%. Those differences are big enough to explain rapid population declines, especially in what are normally long-lived tropical bird species.
More on Tropical Bird Declines
It’s not yet clear what ecological mechanisms underlie these findings. The most obvious explanation is harsher dry seasons that lead to reduced abundance of the insects and fruit that make up a big portion of the diets of many of these understory bird species. Studies in the Northern Hemisphere have pointed to an “insect apocalypse” with massive declines in insect abundance in human-dominated landscapes, but such declines have typically been attributed to agricultural intensification and the widespread use of pesticides. Is it possible that insects are also disappearing from these pristine forests, and that’s what is leading to a collapse in bird populations?
It’s increasingly clear that conserving blocks of apparently pristine forest is not sufficient to ensure the survival of species living in those forests. We need to understand what’s going on under the tree canopy. One of the most striking findings in the new paper is how sensitive species are to relatively subtle changes in rainfall and temperature. That’s likely because tropical understory species evolved in very stable environments, and they are now acting as canaries in the coal mine, warning of the ecological impact of climate change even in the deepest tropical forests.
We need to heed their warning: harness new technologies to monitor these enigmatic species at scale, and prioritize the conservation of landscapes that are going to be most resilient to future change.
