‘Migratory birds declining, lake-dependent resident birds increasing’

Lakes in the city centre have less bird diversity than those on the outskirts due to better habitat conditions, breeding and feeding opportunities

Migratory bird numbers are declining in Bengaluru, while many lake-dependent resident bird numbers seem to be on the rise in the past five years, a new study on the impact of urbanisation on bird populations in Bengaluru has found.

The paper, ‘Relationship between lake area and distance from the city centre on lake dependent resident and migratory birds in urban Bengaluru, a tropical mega-city in Southern India,’ was published in the Journal of Urban Ecology.

Author Harini Nagendra, faculty, Azim Premji University, said Bengaluru’s lakes have gone through multiple phases of creation, renewal, loss, and restoration, impacting bird diversity.

A release said the study is unique because it uses data collected and uploaded to the eBird portal by citizen scientists. The size of the lake area and its distance from the city centre are correlated to the number of bird species that come there. Larger lakes support more species. As the distance from the city centre increased i.e., urbanisation decreases, the number of bird species increases.

The study also found that larger lakes support more diversity as they are likely to have more resting and breeding spaces, feeding resources, and microhabitats. Lakes in the centre of the city have less bird diversity compared to those on the outskirts, probably because of better habitat conditions, breeding and feeding opportunities, it said.

Ravi Jambhekar, Visiting Scientist, Centre for Ecological Sciences, IISc. and the other author, said, “Our study finds that migratory birds are declining but resident bird populations are increasing. We speculate that the decline behind migratory bird populations might be because of some changes in breeding grounds but lake restoration and creation of islands in lakes is helping resident birds. More species-level studies are required to understand the mechanisms behind these patterns.”

“Birds that nest in tree canopies such as cormorants, painted stork, Brahminy kite, and egrets are increasing. This might be because of the availability of nesting sites on the islands created with tall trees. Many of these species are also piscivores and another reason for their increase could be the increase of fish in the lakes,” the release added.

Lesser ducks

The study also found that nine of the ten duck species found in the city are declining, which is reflected in the migratory sites. The decline in duck populations might be associated with urban pressure on their breeding grounds, perhaps from feral dogs and cats or other unknown forms of stress, it said.

“Fish-eating birds such as cormorants and Oriental darters are increasing in the city. Rejuvenating lakes in terms of dredging, creating islands, and introducing fish stock might increase food availability in the lakes, helping these birds maintain stable populations in cities,” the release added.

The other author Kulbhushan Singh Suryawanshi, scientist, Nature Conservation Foundation, said, “Our research shows that the pied kingfisher is declining; they nest in holes excavated on vertical mud banks, and these banks have disappeared as the edges of our lakes are covered in concrete or stones to make walking paths. With careful planning, we can make our urban lakes friendlier for the birds to live in,” he said.