In the misty slopes of the Eastern Himalayas, a crisis is unfolding among the region’s colourful songbirds. A new study spanning 11 years has shown that selective logging of forests fundamentally alters the invisible thermal landscape that birds rely on for survival. Researchers have discovered that as logged forests become hotter and drier, the birds that cannot adapt to these new microclimates are suffering from significant declines in body mass and shorter lifespans. The research provides a missing piece in the puzzle of how wildlife copes with the twin threats of climate change and habitat destruction.
The study by researchers from the Indian Institute of Science, Bengaluru; the University of Neuchatel, Switzerland; and the Nature Conservation Foundation focused on the understorey insectivorous birds of the Eaglenest Wildlife Sanctuary in Arunachal Pradesh, India. While it is well known that cutting down trees harms biodiversity, researchers have long struggled to understand precisely why particular species disappear while others persist.
Did You Know? The canopy of a primary forest acts like a giant umbrella and air conditioner combined. The study found that cutting down trees for selective logging can raise local temperatures by over 2°C and significantly reduce humidity. |
By analysing data collected between 2011 and 2021, the team found that the answer lies in the microclimatic niche, the specific range of temperature and humidity that a species has evolved to tolerate. When the protective canopy of the forest is thinned by logging, the forest floor loses its natural air conditioning, becoming significantly warmer and less humid. For small, sensitive birds, this shift can often be fatal.
The researchers undertook a painstaking mark-recapture program. Over more than a decade, they caught thousands of birds using mist nets in both pristine primary forests and degraded logged forests. Each bird was fitted with a tiny, uniquely numbered aluminium ring, weighed, and released. This allowed the scientists to track individual birds over the years to see how long they survived and how their health fluctuated. Simultaneously, the team deployed temperature and humidity loggers across the forest to create a detailed map of the climate conditions.
They found that logged forests were, on average, hotter and drier than their primary counterparts. Crucially, the study showed that the more a bird’s preferred climate differed from the harsh reality of the logged forest, the worse it fared. Birds experiencing this niche mismatch showed steeper declines in body mass and lower survival rates compared to those in the cool, stable primary forest.
Tropical birds are often thermal specialists, meaning they have evolved to function best within a narrow temperature range. When a bird is forced to live in an environment that is too hot or too dry, it must expend extra energy to regulate its body temperature and stay hydrated. This is energy that can no longer be used for finding food, escaping predators, or reproduction. The study suggests that this chronic thermal stress leads to weight loss, making the birds weaker and more vulnerable to dying. Furthermore, the researchers investigated the food supply by sampling arthropods (insects and spiders). While the types of insects differed between the two forest types, the physical stress from heat and humidity changes appeared to be a stronger predictor of the birds' decline.
While historically, most assessments of habitat degradation have relied on simple abundance counts, the new research focused on demographic rates, specifically survival and body condition, and linking them directly to high-resolution climate data. Doing this has allowed the researchers to identify the mechanism driving the decline. This mechanistic approach will also enable researchers to predict which species are most at risk before they disappear entirely.
As climate change accelerates, understanding the specific thermal safety margins of wildlife is becoming urgent. This research suggests that simply protecting a forest from total destruction is not enough; conservationists must also consider the quality of the microhabitats within it. By identifying which species are most sensitive to heat and moisture changes, forest managers can prioritise protecting dense, old-growth canopies that act as thermal refuges. Furthermore, this knowledge can guide restoration efforts, emphasising the need to replant vegetation that restores the cool, humid conditions vital for the survival of vulnerable species.
This article was written with the help of generative AI and edited by an editor at Research Matters.
