Much-loved songbird threatened by mosquito-borne virus

03 Jun 2024 | No. 2024-19

One of the UK’s favourite garden birds, the Blackbird, is in rapid decline, and members of the public are being asked to help scientists find out why.

For hundreds of years, this familiar bird has been celebrated by poets, painters, and songwriters, but now this popular songster of town and country is in danger of becoming scarcer in many of its former haunts. The British Trust for Ornithology (BTO) has launched a survey to work out why.

Concerns about the common garden bird’s well being have been raised in recent years as birdwatchers have reported fewer Blackbirds in their gardens, especially in the Greater London area since 2020. This decline has been linked to the recent appearance of a mosquito-borne virus affecting the species in England.

First detected in the UK in London in summer 2020, Usutu virus is potentially fatal to Blackbirds and now concerns are growing as the virus appears to have started to spread across south-east England. First identified in South Africa, the virus has been present in mainland Europe for three decades. Its spread has been linked to climate change, and with native UK mosquitoes that can transmit the virus during warmer times of the year.

Now scientists are trying to better understand the extent and spread of Usutu virus and what the potential impacts might be for the UK’s Blackbirds.

Blackbird numbers were already decreasing slightly in Greater London before the arrival of Usutu virus and researchers are keen to see if anything similar is happening in other large cities across the UK, and how this compares to smaller urban and rural areas.

The Blackbirds In Gardens survey will help BTO scientists better understand how Blackbirds use different types of garden, and what factors might influence the risk of disease transmission. Importantly, the survey also seeks to determine how successful the birds are in rearing young, especially at different levels of urbanisation, from rural to urban gardens. 

Usutu is typically spread by bird-biting mosquitoes which rarely bite humans. When it does occur, human infection is often asymptomatic, and there have been no human cases of Usutu detected in the UK to date.

Although the risk to humans from Usutu virus is low, this is the first time in modern history that a mosquito-borne viral zoonosis (a disease which can be transmitted from animals to humans) has emerged in the UK.

This BTO survey is part of a wider partnership project, being run in conjunction with the Animal and Plant Health Agency (APHA), the Zoological Society of London (ZSL) and the UK Health Security Agency (UKHSA). The project, Vector-Borne RADAR, is funded by UK Research and Innovation and Defra to understand the emergence and transmission of mosquito-borne viruses of wild birds in the UK, which are expected to increase as a result of climate change.

Dr Hugh Hanmer, Senior Research Ecologist with BTO said, “Blackbird numbers have been decreasing in Greater London for some time. However, from 2020 they started declining more strongly, which coincided with the detection of Usutu virus. There is now evidence of a wider decline in southern England, not seen in other UK regions. The BTO survey seeks to understand why this change is happening and to identify any link to the emergence of Usutu virus. By better understanding how Blackbirds use our gardens, we hope to halt the declines.”

Dr Arran Folly, senior scientist with APHA and Vector-Borne RADAR project lead said, "Outbreaks of mosquito-transmitted diseases like Usutu virus, which is now endemic in south-east England, are likely to increase in the UK especially as temperatures warm in the wake of climate change. Our Vector-Borne RADAR project is helping to develop a better picture of emerging mosquito-borne viruses and the findings from BTO’s Blackbirds In Gardens survey will be invaluable in building a better understanding of how the virus could be impacting our blackbird populations. I would urge any garden owners to take part and help us keep track of this virus."
BTO is asking for anyone with access to a garden to take part in the survey. Simply sign-up online at Here you will find further information, a simple Blackbird identification guide, and full instructions of how to take part.

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