Bye bird, hello virus

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By Sparrows and other birds are vanishing from cities. One possible fallout could be increased spread of mosquito-borne diseases among humans, reports G.S. Mudur
  • Published 3.11.08
This old police station near Bangalore’s KR Market offers sparrows good nesting sites in the crevices between the thatched roof and wooden beams. Steel-and-concrete structures (like the Bangalore office complex below) don’t. — Bangalore News Photos

In a police station housed in an old building with ageing wooden beams and thatched roof, close to one of Bangalore’s busiest open markets, Melally Giddegowda Venkatesha has discovered an urban oasis for sparrows.

In recent years, ornithologists have observed a sharp decline in house sparrow populations across Bangalore and other cities in India and Europe — from Mumbai, Hyderabad and Nashik to Brussels, London and Berlin.

After a year of counting sparrows at 16 sites in and around Bangalore, Venkatesha has identified the police station building near Krishna Raja Market as the area with the highest density of the birds in the metropolis. Using an identical counting method at each site, he’s observed 616 birds near K.R. Market, compared with a maximum of 280 birds in other central zones of the city.

“The K.R. Market is among the most polluted parts of the city. It has heavy traffic, large crowds, a busy vegetable and grain market,” said Venkatesha, a zoologist with Bangalore University. “But it is also among the best spots in this city to look for sparrows.”

His findings, presented earlier this year in the journal Current Science, have added fresh complexity to the mystery of the decline of sparrows reported by ornithologists and other bird watchers.

“It’s difficult to quantify the decline over time in India because of lack of research on the sparrow population,” said Prashant Mahajan, assistant director of the Important Birds Area division at the Bombay Natural History Society (BNHS). “There is data from the UK, but the decline in India is evident only through observations.”

A study by the British Trust for Ornithology estimated that the sparrow population in London had declined by 71 per cent between 1994 and 2002. Edinburgh, Dublin, Glasgow, Hamburg and Berlin are other European cities that have reported a sparrow decline.

Scientists suspect the decline may be an early sign of predictions by Stanford University researchers that one in four birds is threatened. A study by a conservation biologist, Cagan Sekercioglu, at Stanford four years ago had predicted that about 10 per cent of all bird species are likely to disappear and another 15 per cent could be on the brink of extinction by the year 2100.

The loss could have negative impacts on forests and agriculture worldwide. “Important processes, particularly decomposition, pollination and seed dispersal, will likely decline as a result,” Sekercioglu had said after his study predicting the decline was published in December 2004 in the Proceedings of the National Academy of Sciences.

Disease danger

A new study has indicated that the loss of birds may contribute to the spread of diseases among humans. Biologist Brian Allan, who is pursuing a doctoral course at Washington University, St Louis, in the US has found that high bird diversity appears to protect humans from exposure to the West Nile Virus which, like several other infections, is transmitted by mosquitoes.

Most birds are poor reservoirs — hosts — for the West Nile virus. So when mosquitoes bite birds they are unlikely to pick up the infection.

Where there are more birds to bite, mosquitoes will bite proportionately fewer people, partly reducing their chances of either picking up the infection or spreading it.

“Where many bird species exist, very few mosquitoes get infected, so humans are at low risk,” said Allan. “Where there are more bird species in your backyard, you have lower risk of contracting West Nile fever.”

Allan has analysed bird population density and cases of this viral infection in the US, and his research will be published in a forthcoming issue of the journal Oecologia.

Virologists, however, caution that some birds may be efficient carriers of viruses that can cause diseases in humans. Research suggests that crows, blue jays, sparrows and robins are among reservoirs of the West Nile virus.

The BNHS has now pitched a proposal to India’s environment and forests ministry seeking financial support for a nationwide project to search for sparrows and other common birds that are also harder to spot now than about a decade or two ago.

“The barbet and the brahmani kite are among other common birds of India whose populations also appear to have declined,” said Mahajan. “We need a programme to monitor such changes and understand what is causing them.”

Myriad blows

Scientists have previously proposed myriad theories about why sparrow populations have plummeted — air pollution, lack of nesting sites, reduced food availability, even electromagnetic radiation from mobile phone networks.

Researchers Alfonso Balimori from Spain and Orjan Hallberg from Sweden last year published data that appear to connect the decline of sparrows with electromagnetic radiation. They counted sparrows at 30 sites in the Spanish town of Valladolid each Sunday for four years and compared the density of sparrows to the strength of the electromagnetic field at each of those sites.

Their findings, reported in the journal Electromagnetic Biology and Medicine, suggest that bird density is low in areas with high electric field strength. They argued in their paper that the decline in sparrows might be linked to the increasing number of base stations for mobile telecommunications.

The researchers believe this may also explain why the decline in sparrows is more pronounced around urban centres — where mobile networks are denser and more numerous — than in rural areas.

But some scientists believe that changing city architecture has also played a role in the plummeting population of sparrows.

“Concrete structures provide less spaces for sparrows to build nests,” said Mohamad Dilawar, a zoologist in Nashik who has been observing sparrow populations in Mumbai and Nashik. “And the landscaped gardens we see in cities are like green deserts for sparrows.”

The abundant use of pesticides in landscaped gardens, whether maintained by urban municipal agencies or private complexes, tends to reduce the density of insect larvae available for sparrows to carry for their young ones, Dilawar said.

The new study by Venkatesha suggests that the high density of sparrows in K.R. Market springs from a unique combination — the old building and the open market. The researchers counted more than 15 nests perched on crevices between the thatched roof and the wooden beams of the building. And the grain and vegetables sold outside provide ample source of food for the sparrows.

The growth of supermarkets where grain gets sold in closed spaces and the demolition of old buildings are among factors that appear to be contributing to the decline in sparrow population, Venkatesha said.

“Sparrows are relatively sedentary birds. They don’t travel more than a kilometre or two in search of food,” he said.

“The key to sustaining sparrow populations may lie in providing good nesting sites and a source of food close to them.”