The UK bird population has declined by 44 million since 1966, according to a report by conservation groups.
The study is the first of its kind to give an overall view of how birds in the UK have fared over the decades.
It found that while certain species had increased in number, populations of some common birds had diminished dramatically.
The report, "State of the UK's Birds 2012", was compiled from volunteers' observations of birds since the 1960s.
According to the report, carried out by experts from organisations such as the Royal Society for the Protection of Birds (RSPB) and the British Trust for Ornithology, there are an estimated 166 million birds nesting in the UK compared with 210 million in 1966.
House sparrows were found to be among the worst hit, with numbers down by 20 million compared with the 1960s.
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