Wanted: the grey squirrel, for crimes against songbirds

'Tree rats' raid nests to feast on eggs and fledglings

Squirrels were once thought to live off nuts and berries. But the "tree rats" are blamed for a sharp decrease in some of Britain's best-loved birds by feasting on fledglings and eggs in nest raids.

Research for the British Trust for Ornithology (BTO) shows the first hard evidence linking bird losses to grey squirrels (right). A three-year study by the Game Conservancy Trust revealed that the spotted flycatcher, an endangered woodland bird, suffered huge losses of eggs and fledglings when grey squirrels were not controlled.

Dr Chris Stoate, an ecologist who made the study, said: "The squirrel is the number one suspect. They eat eggs and young birds. Grey squirrel numbers have increased and they live in the woods, where nest losses were at their highest."

Researchers looked at breeding patterns and survival rates of spotted flycatchers in woodland and village gardens in an area of Leicestershire. Numbers of birds, which had risen between 1993 and 2001 when pest-control measures regulated numbers of squirrels, rats, foxes, magpies and crows, fell when the predator control was stopped. A 75 per cent survival rate for nests dropped to just 25 per cent. The spotted flycatcher spends the summer in Britain to breed, then winters at home in northern Africa. It often nests high in trees, safe from foxes, but their offspring are an easy target for grey squirrels.

Dr Stoate said: "Spotted flycatchers have declined by 83 per cent since 1970. If what we found in the area we researched were to happen across the country, we would lose them altogether."

Figures from the BTO in 2005 showed nearly a third of woodland species have declined by more than half in the past 30 years, with the spotted flycatcher one of the worst affected.

The grey squirrel has already managed to drive out the native red squirrel from much of the UK. Bigger and faster to breed, greys have out-competed the reds for food and habitat since they arrived from North America in the late 19th century.