Beloved by Shakespeare and the Beatles, the blackbird and its sweet song have captured the imagination of Britons for centuries.

But now the songbird is facing decline, and the British public has been asked to contribute to a survey by the British Trust for Ornithology (BTO) to find out why.

Their numbers have plummeted in Greater London and southern England at rates not seen in the rest of the UK.

It is thought the bird is suffering from Usutu virus, first detected in the UK in London in summer 2020. This disease is potentially fatal to blackbirds and is thought to be spreading across south-east England. It was first identified in South Africa and has spread across mainland Europe.

The growth of the disease has been attributed to climate breakdown; native UK mosquitoes can transmit the virus when it is warm outside.

Blackbird numbers in London fell 50% in 2020 compared with 2019. Numbers recovered somewhat, but are still about 32% lower now than in 2019. Overall, their numbers have decreased by 18% UK-wide since the 1970s due to factors including habitat loss.

Their numbers were already decreasing slightly in London before the arrival of the virus, so scientists want to know if there is anything else contributing to their decline and what can be done to help.

Dr Hugh Hanmer, a 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.”

Researchers want to see if there are similar patterns in other large cities in the UK and if there are differences between rural and urban areas.

Transmission of mosquito-borne bird diseases is likely to increase as climate breakdown accelerates, according to BTO researchers. Using this survey, they hope to find out how to slow and hopefully stop transmission, so bird populations are not wiped out as new diseases come to the UK’s shores.

It is hoped the survey will help BTO scientists better understand how blackbirds use different types of garden, how successful birds are at rearing young, which factors may influence the risk of disease transmission and how all these factors differ at different levels of urbanisation.

Blackbirds are widely loved because of their complex song, which is very mellow and almost sounds as if it is in a major key. They deliver low-pitched, flute-like verses and are able to mimic each other, and other birds.

The BTO survey is open until October.