Blackbird (Turdus merula)

Dear Readers, every year at about this time, people start to comment on how, after all the hubbub of spring and early summer, their gardens seem empty of birds. I’ve written before about how a combination of birds laying low for their annual moult and the availability of other food means that many of the usual inhabitants of our gardens have gone missing in action, but something rather more worrying appears to be happening to our blackbirds.

Since 2020, blackbird numbers in London appear to have been declining. On an anecdotal level, my garden used to host one resident pair of blackbirds in the summer, with many more in the winter. This year, I have only heard a single male singing for a week or so. At the British Trust for Ornithology (BTO) they are linking the decline to Usutu virus. This is a mosquito-borne virus that is often fatal to blackbirds, and is prevalent across the Channel. For the past few years the disease has been found in London birds, and there are fears that it could be spreading across the south-east. The spread is linked to climate change (sigh) which is favouring the survival and breeding success of the mosquitoes that carry the virus. Affected birds die from damage to the liver and spleen, and from neurological symptoms. There is currently no treatment or cure.

Usutu virus can be spread to humans by mosquitoes, although so far it has been found to be asymptomatic, except in a single case of an immunocompromised person who developed encephalitis. However, scientists have long been worried that other, more harmful mosquito-borne viruses may become more frequent due to warming conditions, and so they have been collecting data on blackbirds in gardens – the survey has been running since May but it doesn’t end until October, so there’s still time to get involved if you’re interested.

Usutu virus originated in South Africa, but has been heading north ever since. In 2001 it was discovered in Vienna, where it started to kill blackbirds. By 2003 it was in Slovakia and Hungary. By 2016 it was found in Belgium, France, the Netherlands and Germany, with the virus causing a  significant decline in the blackbird population as it passed through. Usutu does infect other passerine birds, such as sparrows, robins and starlings, but it seems to be particularly lethal in blackbirds in the UK,  although other parts of Europe magpies, jays and hooded crows have also been badly affected. Like most viruses, Usutu is likely to mutate in ways that might make it more or less lethal and infectious.

Culex pipiens, the mosquito that carries Usutu virus (amongst others) Photo By Alvesgaspar – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2763025

Usutu virus is in the same family of viruses as West Nile virus, another mosquito-borne virus that uses birds as a amplifier/reservoir species (i.e. one where the virus breeds, and which can be used as a source of further infection if a mosquito bites an infected animal). Fortunately for humans, Usutu is nothing like as damaging, but it does flag up the need for constant surveillance – one reason that Usutu was picked up was because there is regular monitoring for West Nile virus in the UK. Public health scientists are constantly on the hunt for which new-to-us viruses might be heading in the direction of the UK, and I imagine it’s a constant battle – it’s not just organisms that harm us directly, but also those that affect crops, livestock and the environment as a whole. It will be fascinating to see what other viruses put in an appearance. We live in interesting times, for sure.