- Leaf warblers are tiny, difficult-to-identify birds in the genus Phylloscopus, which literally means leaf-seekers in Greek.
- A study used open-database records of the birds and an emerging method, Zeta diversity, to find where leaf warbler species are present across Europe and Asia and how snow, artificial lights, and even gravity influence their distribution.
- The migratory birds are not just indicators of environmental change but also a window into the past, holding clues to avian evolution.
A tiny, dull-coloured, restless-looking leaf warbler flies from a garden in Europe to a forest patch in the Indian Himalayas, carrying hidden stories of environmental change and evolution.
“I love this genus,” says Tushar Parab, a Ph.D. scholar at the Wildlife Institute of India.
His recent research explores what drives the distribution of Phylloscopus or leaf warblers — a widespread group of around 80 species — across Eurasia. The study highlights how natural and human-driven forces, from snow cover and elevation to soil moisture and urban lighting, influence where these birds are found. The findings could support conservation planning by understanding their habitat choices and limits to resist environmental change.
These tiny birds weigh just six to ten grams, roughly the weight of a five-rupee coin. Yet, their role in ecosystems is outsized. “They need about 2,000 arthropods a day, but many birds starve in the winter,” says Trevor Price, an evolutionary biologist at the University of Chicago who has studied warblers in India for over five decades.
Found across Europe, Africa, and Asia, these ‘old world’ leaf warblers seasonally migrate between temperate Eurasian breeding grounds and warmer wintering areas in South and Southeast Asia. The greenish warbler, for instance, breeds across Eurasia and when it’s winter there, spends its time almost entirely in the Indian subcontinent. Its muted plumage — olive, brown, yellow, and white — offers little help in identification. Males and females look alike and are constantly in motion, making them a birdwatcher’s aspiration to identify.
“When I first started birding, my friend would say, ‘Leave it and go ahead’ whenever we saw a warbler,” recalls Parab. Instead, he hopped in.
His study, published in the Journal of Biogeography, takes a broader look at leaf warbler distribution across Eurasia using open data from the Global Biodiversity Information Facility (GBIF), covering the breeding season between 2014 and 2023. The researchers studied not just where species occur, but what causes one species to be replaced by another in a region — a concept known as species turnover.
Climate, cities, and subtle forces
The results showed that different warbler groups respond to different environmental factors. For instance, snow cover influences breeding ranges, with species like Blyth’s leaf warbler now nesting at higher elevations in the Himalayas, beyond their usual 2,200 to 3,000 metres, as snow lines recede with climate change, explains Parab.
Artificial night lighting, used as a proxy for urbanisation, impacts species in the P. proregulus group, such as the Pallas’s leaf warbler. “They avoid areas with more light and are shifting breeding ranges. So, there will be a shift in species composition with more urbanisation,” Parab notes. Aerosol concentrations, linked to climate and air quality, may also affect species like those in the P. burkii complex, a group of closely-related bird species.
Other important factors include elevation, soil moisture, and canopy height. A more unexpected player is gravity anomaly — variations in Earth’s gravitational pull in mountainous regions, which can affect birds’ energy use during migration. This subtle force was associated with species turnover in groups like davisoni and burkii.
“It’s important that the study notes that it’s not always climate alone,” says Price. “Other environmental factors play a role too. However, many of these patterns are correlative — we need to look at ecological history and do more on-ground research to understand causation.”
Changes in habitat can influence how species compete for food and territory, and even their chances of interbreeding, says Parab. His study provides a baseline to track future changes in warbler distributions, especially as rare species with narrow habitat preferences face increasing pressure.
New tools, old world warblers
To understand such a diverse genus spread across continents, Parab’s team used Zeta diversity, a method that assesses how species overlap across multiple sites, not just between two. “It’s a promising approach to measure quick shifts in species composition,” observes Price, who was not involved in the study.
However, relying solely on breeding season data from GBIF, built heavily on citizen science data, is a limitation. “We miss a big part of the picture — what happens in the wintering grounds,” says Price. Land-use change for agriculture resulting in habitat loss and the efficiency of pesticides are significant threats. “In Mumbai, for example, you might find a bird that bred in the Himalayas or even Moscow. Each one needs four or five trees to find enough insects to survive. Cut those trees, and that’s one less bird that will survive the winter.”
But wintering records on citizen science platforms are spottier as the birds are comparatively quieter then, and India lacks consistent data collection for longer periods, adds Price.
Adding to the challenge, warblers are hard to identify and get misreported, requiring data cleaning. To improve accuracy, Parab and another team developed an AI-based computer vision model that recognises subtle features such as leg colour, wing bars, and eye rings, in six warblers from the burkii complex as per a study in the Journal of Wildlife Science. The tool has now been expanded to identify all 80 species with up to 85% accuracy, claims Parab. He notes that real-world performance still depends on photo quality, but factoring in species’ geographic ranges could improve it further.
Such AI tools, if integrated into citizen science platforms, could improve large-scale biodiversity data and ease birdwatchers’ frustrations.
Signals from a restless bird
Interestingly, the challenge of telling warblers apart also made them ideal candidates to study bird evolution theory. In a study in Himachal Pradesh, researchers Kristina Fialko and Trevor Price examined whether ambient light influenced the development of slight differences in feather colour among 12 leaf warbler species. Using avian visual models, they found that light conditions alone didn’t explain the colour variation, as it does for animals underwater, suggesting that other evolutionary pressures, such as background habitat colour, are at play.
“The best part is that there are many species in a single genus, and they are sensitive to environmental changes,” says Parab. These unassuming birds reflect the health of forests, insect populations, and even the climate. Yet, little is known about their population status, especially in Asia, clarifies Price.
Not a bird to be overlooked, even if they are hard to identify.
Read more: Understanding the elevational movements of Himalayan birds to improve conservation efforts
Banner image: A green warbler (Phylloscopus nitidus) photographed in Chorokhi Delta, Georgia. Image by Wouter van der Ham via Wikimedia Commons (CC BY 3.0).
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