Avian malaria cited as possible cause of London House Sparrow decline

A study has found that 74 per cent of London's House Sparrows carry avian malaria – more than any other bird population in Northern Europe – and links the intensity of individuals' infections to sparrow decline.

House Sparrow numbers in the capital have plummeted by 71 per cent since 1995, with new research suggesting avian malaria could be to blame. Once ubiquitous across London, the sudden and unexplained decline of the iconic birds led a team from ZSL (Zoological Society of London), RSPB, British Trust for Ornithology (BTO) and the University of Liverpool to investigate if parasite infections were involved.

London's House Sparrows have declined by 71 per cent since 1995 (Carl Bovis).

Researchers collected data between November 2006 and September 2009 at 11 sites across London. Each site was centred around a single breeding colony and spaced at least four kilometres apart to ensure that birds from different groups didn't mix. The team estimated changes in bird numbers by counting the mature males and took tiny blood and faecal samples from sparrows, carefully caught and soon released, to monitor infection rates and severity.

Of the 11 colonies studied, seven were declining. On average 74 per cent of sparrows carried avian malaria – a strain that only affects birds – but this differed between groups with some as high as 100 per cent. However, it was infection intensity (ie the number of parasites per bird) that varied significantly and was higher on average in the declining colonies.

Former ZSL Institute of Zoology researcher and lead author Dr Daria Dadam, now of the BTO, said: "Parasite infections are known to cause wildlife declines elsewhere and our study indicates that this may be happening with House Sparrow in London. We tested for a number of parasites, but only Plasmodium relictum, the parasite that causes avian malaria, was associated with reducing bird numbers."

Professor Andrew Cunningham, Deputy Director of Science at ZSL, added: "Although we found that nearly all sparrows carry Plasmodium, there was no association between the number of carriers and local sparrow population growth. Infection intensity, however, was significantly higher in young birds in the declining populations with fewer of the sparrows monitored in those groups surviving from year to year."

The malaria strains the study identified are widespread and infect multiple bird species. They are, therefore, likely to have been native to the UK, and to House Sparrows long before their numbers started to fall. The parasite is spread by mosquitos, which transfer it when they bite to feed.

It has been suggested that avian malaria will become more common across Northern Europe due to climate change as higher temperatures and wetter weather favour mosquito reproduction, and more mosquitos will help the disease to spread. Researchers think this could be behind the sudden change.