In the most severe cases the lesions caused by the virus in great tits can prevent the birds from feeding or flying and makes them more vulnerable to predators.

在最严重的情况下，大山雀身上由病毒引起的病变，可剥夺鸟儿进食或飞行的能力，使它们更容易受到捕食。

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Mr Ricketts said trust staff had cleared litter after "numerous events" at the site, which is also home to about 60 species of birds including kestrels and long-tailed tits.

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Scientists also recorded countryside great tits and found they sang at a lower pitch and, in turn, city birds found the rural droll difficult to understand, the study found.

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