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As urban heat continues to rise, birds are moving away

Researchers have discovered that as city heat intensifies, bird diversity dwindles. The revelation comes from an expansive study that spanned 336 cities in China, which was conducted by scientists at Zhejiang University and the Cornell Lab of Ornithology.

The findings, published in the journal Science of the Total Environment, reveal a direct link between urban heat, particularly from heat-retaining buildings and paved surfaces, and declining bird diversity.

Urban heat islands 

“The heat-retention characteristic of cities is a well-known phenomenon called the urban heat island effect,” said Frank La Sorte from the Cornell Lab. “Our findings document, for the first time, the direct relationship between bird diversity and the urban heat island effect across multiple seasons.” 

“The heat island effect is not unique to Chinese cities and it is likely that the patterns documented in this study are occurring in other large cities across the globe that have abundant asphalt, steel, and concrete with little green vegetation.”

Moving to cooler areas 

The study authors said birds move to cooler suburban areas, decreasing diversity in the city during the breeding and nonbreeding seasons, but the trend is especially strong during the nonbreeding season. 

Interestingly, the size or location of the city doesn’t seem to make a difference – the decline in bird species diversity remains consistent.

The researchers utilized data from an ongoing study on bird diversity in China. They gauged the surface urban heat island intensity of each city in relation to its suburban surroundings. 

Unexpected diversity loss

“What we did not expect was that diversity loss was even more pronounced during the nonbreeding season,” said study senior author Jiayu Wu from Zhejiang University.

“We predicted that the urban heat island effect would relax the costs of staying warm during the winter, making it less necessary for birds to migrate and resulting in an increase in species diversity during the nonbreeding season, especially in colder cities.” 

Some birds have a higher heat tolerance

Yet, contrary to their predictions, the effect of urban heat on bird diversity showed a consistent negative trend during both seasons in most regions of China.

However, in the Qinghai-Tibet Plateau region, birds seem to be bucking the trend. Here, the urban heat island effect positively correlates with bird species numbers. 

The scientists suggest that birds native to this region might have developed adaptations to its alpine conditions, which could have heightened their tolerance to extreme temperatures. This resilience might be what allows them to flourish amidst the heat of the urban landscapes in the plateau.

Study implications 

The implications of this study are profound, especially as we grapple with the escalating challenges of climate change. Birds, already struggling with other challenges like pollution and shrinking habitats, will face even more dire circumstances as temperatures continue to rise. 

While introducing more vegetation in cities can alleviate some of the heat, it has not proven sufficient in significantly altering the adverse impacts of urban heat islands in China. 

A key area of further exploration would be to determine the exact amount of greenery required to make cities hospitable, not just for the avian world but for humans as well.

More about declining bird populations 

The decline in bird populations is an issue that reaches beyond the borders of China and has profound ecological, cultural, and aesthetic implications. 

Birds play an essential role in various ecosystems, acting as pollinators, seed dispersers, and indicators of environmental health. The observed decline in bird diversity and populations due to the urban heat island effect is just one piece of a larger, more concerning puzzle.

Global decline

Bird populations are dwindling globally, and not just in urban areas. A 2019 report revealed that North America had lost nearly 3 billion birds since 1970, a decrease of almost 30%. 

Similar trends have been observed in Europe and other parts of the world. Various factors contribute to this decline, ranging from habitat loss and fragmentation, climate change, pollution, to the introduction of invasive species and diseases.

Habitat loss and fragmentation

Rapid urbanization and deforestation lead to the loss and fragmentation of bird habitats. This disrupts breeding patterns and food availability, forcing many species to relocate, often with detrimental effects on their numbers.

Climate change

Changes in weather patterns and temperatures can disrupt migratory patterns, breeding seasons, and food availability. The urban heat island effect, as noted in the Cornell study, is a localized example of how these temperature changes can directly impact bird diversity.


Pesticides, heavy metals, and other pollutants can have lethal effects on birds. For example, certain pesticides are known to be highly toxic to birds, leading to immediate deaths or long-term reproductive issues.

Invasive species

The introduction of non-native species can disrupt local ecosystems, leading to increased competition for food and nesting sites. This can drive native bird species to extinction in localized areas.

Human intervention

Direct human actions like hunting and building wind turbines without proper consideration for bird flight paths can also have detrimental effects on bird populations.

Conservation efforts

Recognizing the decline in bird populations, conservationists are working to preserve and restore habitats, implement legal protections, and conduct ongoing research to understand the underlying causes better. Public education campaigns are also underway to involve local communities in bird conservation efforts.


The declining bird populations across the globe are a complex issue with multiple causes. The recent study on the urban heat island effect in China adds to our understanding of this decline but also highlights the complexity of the issue. Coordinated global efforts are required to halt and reverse this decline.


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