By Guy Lynn
Correspondent, BBC News
A second appearance for the Duke of Burgundy could become more common
One of the UK's rarest butterflies has produced a second brood in what conservationists are describing as an extremely rare event.
A second generation of the endangered Duke of Burgundy butterfly has now been spotted in Gloucestershire - the furthest north recorded so far.
Normally the butterfly produces one generation a year, taking to the wing in late April.
But a second brood has been recorded in August in Gloucestershire.
The National Trust said this second generation, documented at Rodborough Common, was highly unexpected.
"I'd never expected to see this," said the National Trust's conservation adviser Matthew Oates.
"I've been butterflying for over 40 years. Butterflies push limits, they really do... this is a really significant moment for one of the Duke of Burgundy strongholds."
While in more southern areas of its range in Europe, the Duke of Burgundy produces a second brood in late summer, in the UK it usually has only one generation each year.
But the spring flight of the butterfly has been occurring increasingly early over the past 20 years, says the National Trust.
Warmer weather has spurred caterpillars to develop into pupae and produce a second brood within weeks rather than emerging the following spring.
A warming climate, which could be behind the earlier emergence of the Duke of Burgundy, may make a second appearance of the butterfly a more common event in the future.
Nearly half the 60 species of butterfly once common throughout the British Isles are on the charity Butterfly Conservation's endangered list. Five have become extinct.
The charity says it is currently most concerned about the Duke of Burgundy, which is characterised by its chequered orange and brown wings.
Butterfly Conservation has reported a sharp decline in its numbers of nearly 60% in the last decade.
"There are probably just 100 sites in the UK where you can find this butterfly," said Sam Ellis, head of regions for Butterfly Conservation.
"Most of these are located in the southern chalk downlands of Southern England.
"But they have suffered worrying declines largely down to loss of their habitats, particularly flower rich grasslands and woodlands where the insects thrive. Many of those have been lost to intensive agriculture, modern forestry and urban sprawl."