The decrease could be due to cold north winds in late May, delaying the arrival of summer migrant birds from Africa.
House martins also rely on buildings for nesting places, so the RSPB is urging people to take care when doing home improvements or renovations during the breeding season.
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Richard Bashford, the survey’s manager, said: ‘We knew that summer migrants have been arriving late this year, so we were expecting numbers to be slightly low, but early indications show that they’re lower than we thought.
‘House martins are incredible birds and you’re extremely lucky if you do have them on your house. I would urge everyone to take part in the survey.’
Make Your Nature Count is a survey of Britain’s wildlife, including birds and animals such as badgers, foxes, moles and deer. For more information, visit www.rspb.org.uk/naturecount
Nightingales are also having a bad year, according to the latest Nightingale Survey, which suggests that we have lost 40% of these summer migrant visitors since 1999.
Chris Holt of the BTO, who has been tracking nightingales, suggest that the nightingale population is linked to deer management.
Mr Holt said: ‘My study provides compelling evidence that increasing deer pressure can have a major effect on nightingale populations, and potentially those of other woodland species, too.
‘When added to the other pressures being faced by migratory birds that winter in Africa, it’s not surprising that population levels are falling rapidly.’
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