But a laboratory result confirmed just that: in April, UK authorities said that Usutu virus had been identified in blackbirds in Scotland for the first time.

Despite climate change pushing temperatures higher, scientists still thought it too cool in Scotland for Usutu to thrive.

The "shock" discovery was a "clarion call that... some risks might be coming here sooner than we thought," said Ferguson, a professor of infectious disease ecology at the University of Glasgow.

"We need to be prepared," she told AFP.

In the wilds of Scotland, scientists are trapping brown-grey culex pipiens mosquitoes -- the main vector of Usutu virus -- to study and better understand how the disease might spread.

They were alerted by residents on the Isle of Arran, southwest of Glasgow, who noticed last summer that blackbirds were dropping dead with strange symptoms.

The birds had a crooked neck, were weak and disoriented, and struggling to feed themselves. A local veterinarian sent remains for analysis and the Usutu diagnosis eventually followed.

- Warning sign -

Named after a river in southern Africa near where it was first identified in 1959, Usutu arrived in Europe earlier this century -- but never ventured as far north as Scotland.

"We thought the climatic conditions were still a little bit too cold. It might be something for the future -- but not now," said Ferguson.

The culex pipiens mosquitoes that carry Usutu are most active and abundant in warm conditions around 25C -- temperatures now frequently reached British summers.

"If you'd asked me 10 years ago what was the risk of ever finding a mosquito-borne disease in Scotland, I would have thought it was minimal and not something that I was going to see in my lifetime," said Ferguson.

"But climate change has really accelerated the pace with which things are changing."

The risk of mosquito-borne diseases like Zika and chikungunya has increased in Europe over the last 20 years, said Emilie Pondeville, a senior research fellow from the Centre for Virus Research at the University of Glasgow.

Mosquitoes need warm conditions not only to survive and reproduce, but also for the virus to multiply inside them. The virus is transmitted when an infected mosquito bites a person or animal.

"There's a balance between the temperature at which the mosquitoes can live, but also the temperature that the virus prefer, and the interaction between both," Pondeville told AFP.

Researchers in Glasgow are conducting laboratory tests to understand how mosquitoes transmit viruses to animals and humans.

Ferguson said Usutu is not considered an immediate risk to public health but acts "as the canary in the coal mine for what might be coming downstream."

Its arrival in Scotland suggests conditions could be suitable for other mosquito-borne viruses.

West Nile virus, a close relative of Usutu, was detected in mosquitoes in Britain for the first time in May 2025. Dengue and yellow fever are also caused by viruses from the same family.

- Bird threat -

For now, what worries biologists most is the impact of these diseases on birds.

Usutu has already caused significant declines in blackbird populations across Europe. The virus also affects owls, raptors, and several passerines.

In some parts of the world, mosquito-borne diseases are already placing extra pressure on vulnerable bird populations.

In Hawaii, the arrival of culex quinquefasciatus -- a tropical cousin of culex pipiens and a vector for avian malaria - has fuelled the decline of several species including the Po'ouli, a small songbird endemic to Maui.

Other threatened species, such as the Akikiki and the iconic I'iwi with its reddish plumage, have sought refuge at higher elevations.

But it could be in vain: as the planet warms, mosquitos and their pathogens are increasingly being found at altitudes above 1,500 meters.