Garden birds are being protected from the effects of climate change by an alien tree, researchers have found.

Turkey oaks were introduced to Britain in the 18th century and have spread across the country, but unlike many invasive species they are thought to be benefiting the native wildlife. Researchers now believe that the species of oak, Quercus cerris, fits perfectly into the native ecosystem because it was a native tree until driven out by an ice age 120,000 years ago.

The tree has been identified as a boon to garden birds because gallwasps lay eggs on its buds early in the spring, and these provide an invaluable feast for species such as blue tits and great tits as they raise their young.

Galls form around the eggs because chemicals on them trick the trees into protecting them, but, being about the size of sesame seeds, they are easily picked off by the birds.

Tits and other types of bird have been driven to lay eggs earlier in spring because of warmer conditions brought on by climate change. Without the gallwasp eggs, few of the young would survive, because the caterpillars that the birds would usually depend on have yet to emerge.

Graham Stone, of the University of Edinburgh, believes that the tits are reverting to behaviour and a food source that was available to their ancestors more than 120,000 years ago. “British birds will flock to trees with lots of galls on and harvest them because they are hungry,” Dr Stone said after carrying out research with the Centre for Ecology and Hydrology. “It’s quite probable that blue tits and great tits are quite used to feeding on it because they have been around for a long time. They were certainly here 150,000 years ago.

“As the Turkey oak reasserts itself in its ancient home, it is helping to alleviate some of the effects of the very modern problem of climate change.”

A study of gall fossils conducted by Dr Stone with researchers at the Netherlands National Herbarium showed Turkey oaks to be present in northern Europe 120,000 years ago. An ice age drove the trees south, and they became trapped behind the Alps, but they were brought back into northern Europe in the 18th century.

The trees reached Britain in 1735, when it was hoped that they would provide the Royal Navy with building materials, but the timber proved inferior to English oak. They became popular in gardens, however. “Everyone who was trendy was having one put in. It quickly became naturalised,” Dr Stone said.

Some people were concerned that the spread of Turkey oaks would disrupt native oak woodland, but he was convinced the two species could complement each other.

Many types of gallwasp depend on the two oaks and at least 11 species have spread naturally to Britain.

Animals and plants could take a very long time to return to their native areas after being driven southwards by ice ages, Dr Stone said. So a proportion of the species reaching Britain to-day were doing so as part of a natural cycle, not man-made global warming.

Alien invaders

— The latest audit of nonnative species in England found 2,721 alien animals, fungi and plants, with a further 988 in Scotland

— The cost of the destruction by nonnative species is estimated at £2 billion each year

— Animals that have suffered because of the introduction of alien species include the red squirrel, muscled out by its grey cousin, and the water vole, which has declined because of the spread of American mink

— The cost of eradicating the plant Japanese knotweed nationwide is estimated at more than £1.5 billion

Source: GB Nonnative Species Programme Board/ Times Archive

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