Private life of the butterfly

April 2005

Pearl-bordered Fritillary: The Pearl-bordered Fritillary is one of Britain’s fastest declining butterflies.
[photo: Martin Warren, Butterfly Conservation]

by Tom Prescott

Now that it is Spring, you will have already seen your first butterflies of the year – probably Small Tortoiseshells and Peacocks flying around your garden, having recently emerged from hibernation.

But how many of you realise that recording such sightings is the foundation upon which the conservation of butterflies is built. Without knowledge of where butterflies occur and how their populations are faring it is impossible to safeguard them. Butterfly populations and distributions are rarely static. These beautiful and magnificent creatures are very quick to respond to changes to their environment, whether these are brought about by changes in climate or changes in the way the land is managed. This quick response makes butterflies ideal indicator species – another reason why recording butterflies is so valuable. Obviously these changes can have a beneficial or detrimental effect on butterfly populations and distributions.

Butterflies that are doing very well in the North-East include the Peacock, Ringlet and Scotch Argus, which are all expanding their range quite dramatically and becoming more numerous. Conversely, it is often the more specialist species such as the Pearl-bordered Fritillary, so-called because of the line of white ‘pearls’ bordering the underwing, which are not faring so well. Between 1982 and 1997 its UK range contracted by over 50%, making it one of the most rapidly declining butterflies in Britain.

The Pearl-bordered Fritillary is predominantly associated with the edges, or open spaces within, woodlands. Adults can be seen from early-May until mid-June, often congregating in sunny, sheltered sites with patches of nectar plants. Bugle is the adult’s principle nectar plant, but they also like other blue or yellow flowers. The sexes are very similar, the females being slightly larger. Males are most often seen flitting and gliding low over the ground, patrolling for females.

Breeding sites are usually dry, sunny, sheltered and south-facing, with short vegetation with a mosaic of light bracken and violets. Like my two children, the caterpillars of the Pearl-bordered Fritillary are very fussy eaters! Their sole foodplant is violets. The caterpillars grow slowly, feeding in spells of warm weather during the summer before hibernating with the onset of cooler weather in September.

It is in March when the caterpillars emerge from hibernation that their parent’s choice of breeding site proves critical. The black, spiny caterpillars need to bask in the weak spring sunshine to raise their body temperature to at least 160C before they can become active, feed and digest their food. At this time of year the air temperature is often around 100C lower than that. But their black colour readily absorbs the heat from the sun and the south-facing, sheltered breeding site provides a very warm micro-climate. Bracken, bare ground or rock is vital, as it readily warms up in the weak sunshine and acts as a sunbed on which the caterpillars can bask.

Individual Pearl-bordered Fritillaries rarely move more than one kilometre in their lifetime, but movements of up to 4.5km have been recorded between colonies across suitable ground. The long-term survival of the Pearl-bordered Fritillary, as with most other butterflies, is more likely if their sites are linked, enabling an exchange of adults between neighbouring colonies. The fragmentation of suitable habitat is damaging, as it makes the surviving populations more isolated and thus vulnerable.

Pearl-bordered Fritillary populations have also declined where deer and livestock have been excluded under woodland regeneration schemes which result in a loss of open space and nectar plants. Light deer browsing or stock grazing is important, though higher levels of grazing, particularly sheep in the spring, can be damaging. Pearl-bordered Fritillary colonies vary in size from a few tens to 1000s of butterflies. Many colonies now only survive under power-lines, as they benefit from the 7-10 year cyclical scrub clearance beneath the wayleaves.

Almost all the Pearl-bordered Fritillary colonies in Grampian occur in Deeside upstream of Banchory. A survey last year co-ordinated by Butterfly Conservation Scotland failed to find Pearl-bordered Fritillary at a number of previously known colonies, yet discovered two new large ones. Its real status in Grampian is still unclear.

Further fieldwork is planned for 2005, and we would welcome your assistance. If you want to get involved, Butterfly Conservation Scotland is holding a Pearl-bordered Fritillary workshop in Aboyne at the end of May. The day is aimed primarily at local residents who wish to know more about the species and would then be willing to get involved in survey and monitoring local colonies. No prior knowledge is required.

Twenty-five of the 33 species of butterfly that regularly occur in Scotland also occur in the North-East. Like the Pearl-bordered Fritillary, the caterpillars of all of these have specific foodplants and habitat requirements, and fairly strict and often short flight periods. Knowing these fundamental facts is invaluable when you are trying to identify butterflies.

The distribution and status of even the commoner species in our area is often poorly known and we really do need your help. Butterflies for the New Millennium (BNM) is Butterfly Conservation ’s project for recording distribution. It has been immensely successful to date – since its launch in 1995 10 000 volunteers have collected over 1.9 million butterfly sightings. Data from the first five years of BNM were used to produce The Millennium Atlas of Butterflies in Britain and Ireland, (2001) but this book is only the start.

The distributions of most species are changing rapidly and it is vital that more recorders are recruited. You don’t have to be an expert and even recording in your garden, local park or nature reserve can be a big help.

Standard recording forms are available to note down any butterflies that you see, together with the date, name and grid reference of the place where they were seen.

The Butterfly Conservation Scotland is running workshops to give you the basic skills to go out and start looking for, identifying and recording butterflies in the area. It can be great fun. Can you think of a better way of whiling away a sunny afternoon in your garden or out in the countryside counting butterflies?

Dr Tom Prescott works from home in Kingussie as conservation advisor for Butterfly Conservation Scotland. He previously worked for the RSPB for 14 years, primarily on reserves in Scotland. This was preceded by gaining a PhD in structural engineering.

This is an article from the April 2005 edition of Leopard Magazine. To read much more like this every month, subscribe to Leopard Magazine.