Millions of songbirds are vacuumed out of trees and killed each year during the nocturnal Mediterranean olive harvest, researchers have warned.

Vast numbers of legally protected birds from central and northern Europe seek refuge in the Mediterranean basin during winter months.

At night they roost in olive bushes, just as intensive harvesting machines start stripping trees of their fruit (the harvest runs from October to January).

The light of the machines dazzle and disorientate the birds, who end up being sucked into them “on a catastrophic scale”, researchers warn. There can be as many as 100 dead birds in each harvest trailer.

In Andalusia in Spain, 2.6 million birds are killed by harvesting tractors every winter. Iconic British birds like robins, greenfinches, warblers and wagtails are among the highest causalities.

“The machinery is perfectly fine if used during the day, as birds are able to see and escape while they are operating,” lead researcher Vanessa Mata from the Portugal-based Research Centre in Biodiversity and Genetic Resources told The Independent.

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“However, during the night they use very strong lights which confuse the birds and lead to their death as they are ‘sucked in’ by the tractor.”

Trees are stripped at night because cooler temperatures help preserve the olives’ aromatic flavours, according to the correspondence, which is published in Nature.

Ninety-six thousand birds die in Portugal every winter as a result of this intensive farming technique. France and Italy also carry out the practice but specific numbers are not known.

Martin Harper, the RSPB’s director of conservation said, “Numbers of farmland birds in Europe have plummeted by 55 per cent over the last three decades and this is another shocking example of how modern agricultural practices are impacting our bird populations, including some UK species passing through the region.”

Andalusian officials have recommended the practice stops but unless legislation is passed in the next few months the “massacre” will start again in October, researchers warn.

France, Italy and Portugal are yet to take any action.