Noisy but admired by many, the sparrows used to be a regular sight in London’s gardens, but their number has dropped 71% since 1995. Now, a new study suggests that avian malaria might have a part to play in the decline.

Credit: Nmahieu (Flickr)

The “sudden and unexplained decline of the iconic birds” inspired a team from Zoological Society of London, the Royal Society for the Protection of Birds, the British Trust for Ornithology and the University of Liverpool to investigate what was going on.

Up to 74% of the city’s house sparrows were carrying avian malaria, according to their research. That’s more than any other bird population in Northern Europe. While it is a strain that only affects birds, it is still cause for alarm.

“Parasite infections are known to cause wildlife declines elsewhere and our study indicates that this may be happening with the 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,” said Daria Daram, lead author.

Researchers searched for parasites for three years by taking blood and fecal samples from sparrows in different areas, ranging from Enfield in north London to Sutton in the south and Fulham in the west. They were centered around a single breeding colony and spaced at least four kilometers apart.

In some areas, 100% of birds were infected with the avian malaria parasite and, when the birds were counted, many juvenile sparrows did not survive the winter. The disease is spread when mosquitoes bite birds and feed on their blood. It can lead to infections that can be fatal to the birds. At the very least, it is adding more pressure on sparrows, making it harder for them to survive.

The research follows other studies that have ruled out domestic cats as a cause of falling sparrow numbers, while casting doubt on whether sparrowhawks are responsible. Some experts also suggested that, because sparrows fail to move very far, populations may be becoming inbred.

“Exactly how the infection may be affecting the birds is unknown. Maybe warmer temperatures are increasing mosquito numbers, or the parasite has become more virulent,” said Will Peach, head of Research Delivery at the Royal Society for the Protection of Birds.

With a changing climate, researchers expect that avian malaria will become more widespread across Northern Europe, thanks to higher temperatures and wetter weather, both of which affect mosquito reproduction. This could be linked to the change with the sparrows, they said.

“It has been hypothesized that Plasmodium prevalence will increase across Northern Europe due to climate warming, and that climate change will influence avian malaria infection rates through increased parasite and vector abundance and altered mosquito distributions,” the study reads.

The study was published in Royal Society Open Science.