Birds and berries to be surveyed

Song thrush with a red berry (c) John Harding / BTO A pernicious pick?

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Gardeners and birdwatchers are being asked to help identify birds' berry-eating habits.

The British Trust for Ornithology (BTO) study is the first large study of how birds use garden berries and fruits.

While it is known that berries attract birds, little is understood about which berries different bird species favour.

Scientists are concerned that by preferring certain berries, birds may be spreading the seeds of invasive plant species, such as cotoneaster.

The bounty of berries that autumn and winter produce can be an important source of food for many birds, such as thrushes, starlings and wintering warblers.

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They offer sugars and easily digestible nutrients during a time when food can be scarce.

The plants that produce the berries then rely on the birds to disperse the seeds hidden within this nutritious flesh.

Garden berries are important as they are usually available later in the year and often untouched by other birds.

The mistle thrush will aggressively defend holly berries all winter, for example.

But little is actually known about which garden berries and fruits are being eaten by which birds and when.

"There is no evidence for how important garden berries are compared to those in the wider countryside," said Mike Toms, head of garden ecology at the BTO.

The study will also aim to discover whether the birds are choosing to eat berries from native garden favourites such as holly, rowan and elder, or popular non-native garden species such as cotoneaster, pyracantha and berberis.

Choosing to eat berries from a native or non-native species could depend on the colour or availability.

This is of particular concern as cotoneaster is on the UK government's Department for Environment, Food and Rural Affairs' list of non-native invasive species, alongside Japanese knotweed and Himalayan balsam.

"The worry is this invasive species [cotoneaster] could become further established in the wider countryside," Mr Toms told BBC Nature.

The Birds and Garden Berries Study launches on 30 September and runs until 30 March 2013.

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