Anthropogenic changes threaten survival of Eastern Himalayan birds

Insectivorous birds found in the understorey of the Eastern Himalayas are under threat due to habitat degradation, a new study from the Indian Institute of Science (IISc) has found.

Researchers at the Centre for Ecological Sciences (CES) studied how changes in forest microclimates after selective logging influence the survival of wild bird populations in the Eaglenest Wildlife Sanctuary, Arunachal Pradesh, over 10 years (2011-2021).

The team tagged birds with lightweight aluminium rings and revisited the same sites annually to track their survival and changes in body mass. They paired this dataset with temperature-humidity loggers placed in both primary and logged forests, to estimate how understorey insectivorous birds – those that live below the canopy – adapt to microclimatic changes. “Using these long-term data sets, we can better understand why some species survive after logging while others decline strongly,” says Akshay Bharadwaj, a former Master of Science student at CES and corresponding author of the study.

Overall, the team found that logged forests are consistently hotter and drier during the day and colder at night – in comparison to primary forests – exposing birds to stressful fluctuations due to loss of the forest canopy. These conditions, the scientists say, could intensify with climate change, especially in the Eastern Himalayas where bird species are thermal specialists – uniquely adapted to stable climates.

Their findings reveal that these birds, which utilise very different primary and logged forest microclimatic niches, are most adversely affected in a logged forest – they experience a decline in body mass, and steep declines in long-term survival. “Species that can still find microclimates in logged forests similar to their original forest homes are surviving after selective logging. It is those which can’t match their old conditions that face steep declines,” adds Bharadwaj.

“Being in the field studying these fascinating animals is always a thrill. We work in a relatively remote part of Arunachal Pradesh, collecting field data under sometimes challenging field conditions – rain, leeches, and elephants,” says co-author Umesh Srinivasan, Assistant Professor at CES.

Based on their findings, the researchers suggest that conservation strategies should prioritise preserving primary forest across elevational gradients. “In degraded areas, we can consider microclimatic remediation, such as creating shade covers or supplementing water sources to mimic original microhabitats to support vulnerable species,” explains Bharadwaj. “The impacts of forest degradation will impact food chains that are part of larger ecosystem processes.” A drop in the number of insectivorous birds can lead to an increase in insect numbers, which in turn can affect ecological stability, he adds.

The study highlights the significance of understanding why certain species of birds are declining after logging, and how microclimatic niches in disturbed habitats influence population dynamics. “Long-term datasets are crucial for this, and we are continuing to collect these data to try and plan effective conservation measures for these bird species,” explains Srinivasan. “As the climate warms, the persistence of microhabitats will be crucial for many species to remain resilient to climate impacts.”