Once a common sight around London, house sparrow (Passer domesticus) populations have been declining for decades; they’re down 71% since 1995. Now, researchers believe they know why: a mosquito-borne disease called avian malaria.
Scientists collected 3 years of data from 11 sparrow colonies around London where the birds breed. They counted raw numbers of birds each year, and collected blood and excrement from a number of individuals.
Seven out of the 11 colonies were losing birds, and roughly 74% of the sparrows carried avian malaria (Plasmodium relictum). That’s the highest rate of infection with this parasite seen in any wild bird population in Northern Europe, the researchers report today in Royal Society Open Science. Avian malaria may be causing declines in Western Europe, North America, and India as well, the team says.
Similar to other forms of malaria, avian malaria is spread when mosquitoes bite birds and feed on their blood. The disease can lead to infections that can be fatal to the birds, and they can pass the infection on to their offspring. Most sparrows carried the parasite, but the quantity of parasites found in each bird’s system was significantly higher in declining populations, especially in younger birds, the team found.
The scientists don’t know why avian malaria is particularly prevalent in house sparrows, but they say further research may provide clues.