The survival of hen harriers “hangs by a thread”, conservationists warned as figures showed just seven pairs of the bird of prey nested successfully this year.

According to the annual survey of breeding hen harriers, only 12 pairs even attempted to nest in England.

While the number of pairs successfully raising their chicks to fledging was one more than last year, when just six nests were successful, the figure for 2010 is half the number which nested successfully three years ago.

The RSPB said there was sufficient habitat for 300 pairs of the rare bird of prey in England, but that persecution of hen harriers remained “devastatingly common” despite the birds having full legal protection. Of the successful pairs, five nested on the United Utilities estate in the Forest of Bowland in Lancashire, the remaining stronghold of the bird in England.

Mark Avery, RSPB director of conservation, said: “Persecution, associated with land managed for driven-grouse shooting, remains the main reason for the hundreds of missing pairs.

“Even though these birds now have the full protection of the law, the persecution of birds of prey remains devastatingly common.”

Hen harriers come into conflict with upland grouse shoots as they eat the game birds.

The RSPB is calling on the government to address the problem of illegal persecution, and as a first step wants the coalition to confirm that the future of the National Wildlife Crime Unit is secure.