Record breaking year for Swallowtail Butterfly

2017 saw a significant increase in recorded numbers of the Swallowtail Butterfly population in multiple key locations across the Broads National Park. This rare and Broads specific-species has seen its highest recorded population levels since 2011.

The Swallowtail Butterfly species is dependent on milk parsley, the plant upon which they lay their eggs and eat as their sole food source when caterpillars. The Broads National Park is a UK sanctuary for milk parsley, which is a tall umbellifer that loves wetlands supplied with chalky water. The plant also depends on open fen as well as the correct water level and management to prevent scrub growth.

The encouraging new figures represent the hard work of the Broads Authority, Biodiversity Partners and landowners who together successfully achieved their goal of 400 hectares of restored open fen in 2010, a figure which was comparable with its extent in 1946. The challenge has since been to maintain this pioneering project with the rewards of this work being seen in the figures for 2017.

The collection of Swallowtail butterfly data is achieved through the dedication of volunteers who walk set routes, which are known as transects. The transects run through nature reserves and volunteers record butterfly numbers as they pass through them. The transect scheme is a national effort, forming a major part of the UK Butterfly Monitoring Scheme administered jointly by Butterfly Conservation, The Centre for Ecology and Hydrology and the British Trust for Ornithology.  Additionally there is a network of recorders who send in records of Swallowtail sightings to the County Butterfly Recorder, which gives information as to their movements across the county.

Alan Dawson, the Transect Coordinator of the Norfolk branch of Butterfly Conservation said of the increase,

“Without all the work put in by Broads Authority, RSPB and Natural England this environment would be rapidly lost.  So when the numbers are good, it means that the management of the fens is working and the milk parsley is doing well.”

Weather conditions have played a crucial role in the Swallowtail success story. The growth of the milk parsley, the flight pattern of the Swallowtails and the Swallowtail pupae lying attached to the base of reed stems can all be affected by poor weather conditions. The good weather of 2016 and 2017 has given the butterflies an opportunity to thrive without the threat of harsh conditions. The swallowtail used to appear in late May and June, but in recent years a sizable second brood in late summer has given Swallowtail Butterflies a second chance at increasing their numbers.

Senior ecologist, Andrea Kelly said,

“This summer provided good weather conditions for flying butterflies and some days you could see literally hundreds of these big yellow and black butterflies zooming over the rich fen vegetation finding mates and searching for a drink of nectar.”

With the continuation of vital fen management and landowners creating favourable wetland habitats across the marshes, rich in milk parsley, the Broads Authority hope to ensure that the Swallowtail Butterfly population will be resilient to a changing climate.

Swallowtail at Strumpshaw © Broads Authority

Thursday 16 November 2017