The silent House Sparrow killer


There are many culprits behind House Sparrow decline, but one of the more intriguing is a virus, coined “The Avian Malaria”. It’s caused by a parasite called Plasmodium relictum, which is widely dispersed and spread amongst birds by its vector, mosquitos.


Once the parasite has invaded the House Sparrow’s body, it will live secretly inside of its living cells, undetected by the bird’s white blood cells and so preventing it being fought off. It then asexually reproduces, multiplying until the cell ruptures, and the viruses explode into the blood stream, transported to new host cells, which it breeches and reproduces in again, and the cycle continues.


Effects of the virus include lethargy, tiredness, anorexia, susceptibility to infection, depression, etc, however when very young, old, or ill birds are infected the virus can be enough to kill the individual. Sadly, due to how fast it spreads, and how apt it is at multiplying undetected, it’s hard to determine whether a bird has it or not until it’s too late.


Thanks to their sedentary nature (most male House Sparrows don’t travel more than 100m in their lifetimes) sometimes 100% of a House Sparrow population can have the parasite. This is because, when populations stay isolated and don’t mix, the genetic diversity gets run down, and viruses and infection have a much heavier toll.


Unlike other birds (e.g., several penguin species) Avian Malaria isn’t the main cause of House Sparrow decline. However, it’s still something that should be addressed, as even when it doesn’t kill a bird itself, it weakens them to the point where something as small as weather can finish them off later down the line.



And unfortunately, the parasite is on the rise. It’s all thanks to the elephant in the room in the world of wildlife and the environment – Climate Change.


As the parasite is dispersed through mosquitos, the much milder, warmer Winters that’d normally cull the mosquitos leaves an unnatural amount that survive. And with more Mosquitos comes more of their microscopic passengers.


Warmer temperatures globally also now mean the parasite is breaking new ground, spreading to Norths and Souths it’d previously have been unable to survive in due to their harsh climates (which is why penguins are being hit so hard – they have no natural resilience to the virus).


While there isn’t known to be a ‘cure’ as such, our cheerful little House Sparrow neighbours are amazingly figuring out ways to fight it. Before humans!


House Sparrow nests hum with ticks, mites, fleas, and other pests. So, over the years they’ve incredibly learnt how to self-medicate them. They’ve been observed foraging for greenery such as Wild Carrot and Wormwood, known for their insect-repelling properties, which ward off invertebrate pests.


But, during an outbreak of Malaria in India, they took it one step further. It was recorded that House Sparrows started foraging for sprigs of the Flame Tree, pleating them into their nests. Following research on the plant, it was found that this plant had anti-malarial properties. So, somehow the Sparrows “knew” this, and through lining their nests with it, protected their babies from it.


A roadside verge covered in frothy white Wild Carrot

Have our British Sparrows evolved similar skills? We don’t know, but it wouldn’t be surprising, as House Sparrows are actually one of our more intelligent birds here, why’d you think they became so common to begin with?


Although there’s no direct way you can help protect your Sparrows from Avian Malaria, you can help reduce its impact on them by keeping them healthy, keeping them fed with invertebrates, leaving patches of your garden to go wild, and installing nestboxes, giving them the best chance, they have to fight the virus.


Colony nestboxes are ideal for House Sparrows.

As always, my email and social media(s) are open if you’re curious to know anything else.


Thank-you for reading!