One of the UK's rarest birds has enjoyed its most successful year on record, despite being hit by the harsh winter last year, conservationists said.

Monitoring revealed the number of bitterns "booming" – the loud call made by male birds to attract a mate – reached 87 this year, up on the previous record of 82 males heard last year.

In Somerset, 14 males were recorded this year in the Avalon Marshes to the west of Glastonbury, seven more than in 2009.

The Marshes includes the Royal Society for the Protection of Birds' Ham Wall nature reserve and Natural England's Shapwick Heath National Nature Reserve.

Experts said they were "amazed" the bittern, a bird which feeds in shallow freshwater and is vulnerable to the kind of long freezing spells seen last winter, had come through the winter so strongly that numbers increased.

The species of heron became extinct in the UK in the 1880s and recolonised the country in 1911. The population increased up to the 1950s, but by 1997 the bird had reached its lowest numbers since the 1920s, with 11 booming males recorded.

But a monitoring and research programme, which led to improved management and restoration of the bird's reedbed habitat, along with the creation of new nature reserves, has boosted numbers, the RSPB said.

Dr Mark Avery, the RSPB's conservation director, said: "The bittern is perhaps the best example we have of the value of targeted conservation action. With funding from government, the European Commission and the business sector, the bittern has gone from strength to strength.

"However, with dire predictions of swingeing cuts to Government budgets, we remain deeply concerned about the future for the bittern."

Conservationists were particularly surprised that the birds came through the winter so strongly because their food of sprats would have been unavailable as the shallow freshwater areas they feed in froze in the icy temperatures.

Wildlife groups were giving bitterns supplementary food during the harshest winter for 30 years and offering advice to people on putting out sprats and fish for the birds.

Despite the hard conditions, researchers from the RSPB and the Government's conservation agency Natural England recorded bitterns booming this year in reedbeds in a number of English regions.

The overall number is one less than in 2009, but the bird's range has expanded with males recorded at 47 sites – up on 43 last year.