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Record numbers of rare bitterns seen at a Kent nature reserve are thought to be a result of the harsh winter this year.

The RSPB said 11 bitterns were seen on one day at its Dungeness site - more than double last year's total of five.

Water froze on small ponds and reedbeds and the birds were forced to gather in one place to feed, the charity said.

Site manager Bob Gomes said it was normal to see more birds at Dungeness during the winter, but the high numbers this year were "unprecedented".

Weather conditions had since improved, but the bitterns had got used to their new feeding areas and had stayed there, Mr Gomes added.

Visitors have regularly watched up to five birds at once this winter, and the charity has started running "bittern safaris".

Mr Gomes said: "This clearly demonstrates the potential of Dungeness as a future British hotspot for this endangered species."

Figures for bitterns at the site showed that only one or two birds were seen each winter until 2002. After 2002, numbers increased to three or more. Eight were counted in January 2009, and five were seen during winter last year.

Numbers of birds normally increased at Dungeness at this time of the year as they escaped the harsher winters of northern Europe, Mr Gomes explained.

But prolonged snow and frost in December had seen a further rise on the Dungeness peninsula.

Numbers of Bewick's swans increased throughout December reaching a peak of 146 - the swans normally feed on nearby Walland Marsh in the day and return to Dungeness at night to roost.

And "nationally-important" numbers of wigeon, shoveler and white-fronted geese were all recorded at Dungeness that month, according to RSPB statistics.