How urban birds respond to heat and green cover

What determines where birds live in a city?

If you guessed green spaces, you’d probably be right. But heat and the presence of heat islands in the city seem to be equally, if not more important factors that drive bird distribution in Bengaluru.

Using citizen science records from eBird and satellite data, scientists from the Indian Institute for Human Settlements (IIHS) explored how thermal stress and green cover affect 19 bird species across Bengaluru. The findings show that all these species avoid the hotter areas of the city. The work also shows that green cover was preferred by some species, while others are adapting to and even exploiting the concrete jungle.

Mapping bird occurrences across the city

The researchers used satellite-derived data to measure vegetation cover and land surface temperatures over Bengaluru to create a detailed map of the city’s green cover and “hot” and “cool” zones. This map was then overlaid with a 2 km X 2 km grid of cells of presence/absence data on 19 bird species derived from over 43,000 eBird checklists reported across seven years, from January 2014 till March 2021. Although Bengaluru hosts a remarkable 379 species of birds, high-quality data were obtained for only 19 species, which were chosen to ensure robust analysis.

Using Bayesian statistical analysis, the team explored how green cover and heat affected bird species presence across Bengaluru. “Bayesian analysis, unlike traditional methods, incorporates prior knowledge and updates estimates of parameters such as a slope of a covariate in a regression as new data becomes available,” says Jagadish Krishnaswamy, Dean of the School of Environment and Sustainability (SES) at IIHS and the corresponding of author of the study. “It allows us to predict where birds might be present based on environmental variables, offering nuanced insights into species-specific habitat preferences,” he says.

The results on the effects of heat were unambiguous — all 19 bird species showed a negative association with rising surface temperatures. Hotter areas in the city consistently recorded fewer sightings of birds, regardless of species.

“We know that other studies have shown similar patterns in tropical areas,” says Ravi Jambhekar, one of the authors of the study from IIHS. “But in temperate regions, the effects of heat islands can be variable; for example, some birds in North America congregate in city centers to escape the low winter temperatures,” he says.

In Bengaluru, however, it is clear that the rapid urban expansion, loss of vegetation, and rise in built-over surfaces, are creating a cityscape that is becoming increasingly hostile to bird diversity.

Not all green is equal

While heat stress uniformly deterred birds, the role of green spaces was more complex. Some species such as the black-winged kite, black drongo, Indian nightjar, and coppersmith barbet preferred greener zones; however, others like the common myna, barn owl, black kite, Asian koel, and Alexandrine parakeet were more abundant in less vegetated, urbanised areas.

This variation may be due to feeding behaviour. Urban exploiters like the barn owl have adapted to hunt rodents that forage in the garbage dumps scattered through the city. Common mynas and black kites scavenge readily available anthropogenic food.

Amongst the species that prefer greener areas, the black-winged kite, is an active hunter reliant on intact green patches for finding prey. Similarly, the coppersmith barbet, which is frugivorous, needs these green areas to survive.

“Such relationships (like those between green cover and bird presence) are very difficult to predict in advance because our knowledge of urban birds and their requirements are at a very early stage,” says Ashwin Viswanathan, a researcher at the Nature Conservation Foundation with experience in studying birds. “Results such as the correlation of higher black-winged kite presence with green cover are superficially surprising because this is a species associated with ‘open’ landscapes, but perhaps not surprising because even open landscapes in a city like Bengaluru are strongly associated with campuses and similar areas that also have green cover,” adds Viswanathan, who was not associated with this research.

An unexpected finding was the neutral and negative responses to green areas exhibited by the Alexandrine parakeet and the Asian koel, which are fruit-eating species and logically, should be heavily reliant on vegetation.

“This is likely because many urban green spaces are dominated by exotic species that may not provide suitable resources,” says Jambhekar. “The green cover estimated from satellite data doesn’t differentiate between a patch of native vegetation and a plantation of exotic species,” he explains.

“Satellite based green indices are likely to be higher for areas with tree cover although some bird species need more open habitats with grasses and shrubs,” adds Krishnaswamy. This nuance complicates the assumption that greener automatically means better for biodiversity.

The study, therefore, urges caution in equating green cover with ecological value, as Bengaluru’s greenery often includes decorative palms and exotic tree species, which offer little in terms of food or shelter for native Indian birds.

For urban planners, the findings provide actionable insights. Bengaluru will need to not only preserve its existing green spaces but must also plan to expand them by using native plants in landscaping projects to connect fragmented habitats with green corridors. Overall designs for public spaces will now need to consider both human and wildlife comfort, especially for heat mitigation.

“I found it very exciting to see that citizen science data generated by the public has been used to better understand the impacts of urban planning on birds. This study and similar studies can be very useful for urban planning to maximise biodiversity that can then provide joy and pleasant living conditions to the public,” says Viswanathan.

This work showcases the power of citizen science. By harnessing data from amateur birdwatchers on eBird, researchers could map city-wide biodiversity patterns and build models with real-world implications. It is a reminder that science does not just happen in labs, but can be crowdsourced from backyards, balconies, and city parks.

Could green city spaces be a conservation opportunity?

A byproduct of the vegetation mapping analysis in this study is that it identifies green patches within the city that could play a dual role. On one hand, they function as refugia for urban wildlife, including birds. On the other, they also benefit humans by acting as carbon sinks and providing ecosystem services such as nutrient recycling, ground-water recharge, and heat mitigation.

This synergy opens up a different type of conservation strategy — other effective area-based conservation measures (OECMs). Unlike traditional reserves, OECMs recognise the biodiversity value of places, such as campuses, parks, and urban farms, where conservation is not the primary land use goal.

In Bengaluru, the Indian Institute of Science and Gandhi Krishi Vigyan Kendra campuses stand out as OECMs. These places not only harbour threatened bird species, but also act as heat dispersers, offering cooler microclimates within the city to both humans and wildlife. Recognising and protecting such areas could bolster both urban biodiversity and climate resilience.

The authors advocate leveraging India’s emerging green credit programme to fund the maintenance and conservation of these OECMs. Designating more OECMs and integrating them into city planning could be a powerful step toward more liveable cities.

Other patterns in bird distributions and further research

Apart from exploring the effects of heat and green cover on birds, the study also investigated differences in the distribution patterns of pairs of species that use similar resources.

For example, the common kestrel, a migratory raptor, was largely restricted to the city’s cooler outskirts, while the resident shikra, a similarly-sized predator, was more widespread, even within the city centre.

Amongst frugivorous birds, the red-whiskered bulbul and white-cheeked barbet were widely distributed, while the red-vented bulbul and coppersmith barbet were more localised, appearing in specific city pockets. The nectar-feeding purple sunbirds and purple-rumped sunbirds were both concentrated in greener areas with overlapping territories, where nectar-rich flowers could be found.

The authors speculate that these contrasts among the species pairs may stem from differences in adaptability, tolerance to human disturbance, and reliance on human-derived food sources. In addition, these differences could also be due to competition, nesting requirements, or local vegetation types.

“A weakness of the study is that the data uploaded by birdwatchers within the city may be a little patchy with all areas not equally well represented, as is the nature of such data,” says Viswanathan. This is apparent in the fact that the 19 species of birds included in the study do not include one of the most common urban birds — house crows and rock pigeons.

Jambhekar agrees with this assessment and adds, “Although there was enough data on rock pigeons, but the data on other closely related pigeons from the city was patchy; therefore, we were unable to analyse the data for these closely related species pairs and make comparisons. But the number of observations for crows, especially in certain areas in Bengaluru, are few; this is in contrast to what I have observed in other cities such as Mumbai and Delhi.” If the sightings of crows in Bengaluru are indeed dropping, it could signal a major issue with the population of these birds in the city, which has already lost most of its house-sparrow population. However, Jambhekar adds a rider to this, saying, “This is just an observation that I have seen. No formal analysis has been done to prove this trend yet.”

“The best way to address the issue with patchy data, especially if this information can support urban planning, is that going forward, birdwatchers in the city should come together to do a Bengaluru bird atlas that will involve uniform sampling in space and time. Delhi is currently doing an atlas of this sort,” says Viswanathan.

Read more: Increasing summer temperatures impacting birds in Indian cities

Banner image: A kestrel perched on a pole in Bengaluru. These birds are more common in the outskirts of the city. Image by Jagdish Krishnaswamy.