One of our brilliant volunteers, Richard Marriott, shares his experience with counting pearl-bordered fritillary and the exciting year they have had.
Counting pearl-bordered fritillary at Crannach
I first got involved in surveying and recording at Crannach in 1991, six years after friends bought the site and formed the Crannach Management Group. Since then I have been recording plants, molluscs, adders, butterflies and other insects, and of course birds. I continued surveying and recording as a RSPB Scotland volunteer after RSPB Scotland acquired the reserve in 2011. Each year I undertake monitoring of various species including green shield moss, twinflower and pearl-bordered fritillary.
The interest in pearl-bordered fritillaries at Crannach began in 2005, after I attended a talk by Tom Prescott of Butterfly Conservation Scotland on the conservation status of the species. After Tom’s talk I decided to set up an annual monitoring transect at Crannach to find where pearl-bordered fritillaries were and to monitor their numbers. The difficulty was that they occur mostly in the two narrow glens which are separated by a high ridge; so, what evolved was a transect going up one glen then a very steep 230m climb up the ridge through deep heather and down into the second glen, then following it down and back to the start. Latterly the climb has proved too much for my ageing knees and we have split the transect. Two other RSPB Scotland volunteers, Susan and Martin, survey one glen, and I survey the other. We cannot be sure the numbers from each annual survey are total population counts for the reserve, but they do show us where the butterflies are and give some indication of the health of the population on the reserve.
We aim to undertake the survey when the maximum number of pearl-bordered fritillaries are on the wing, and when conditions are sunny, with temperatures above 14ºC. This is usually in late May, but obviously varies each year depending on the weather. Some years it has proved impossible to find a suitable day to undertake the survey.
This year we first heard of pearl-bordered fritillaries emerging on Deeside on 30th April - a very early date! Was it going to be an early season? Then poorer weather set in and we were left wondering. The forecast for late May looked poor, with the only possibility for trying the survey on the 23rd, so we decided to make an attempt.
We arrived at the reserve mid-morning (still only 12ºC but slowly getting warmer) and when half an hour later we saw the first butterfly on the wing we set off up our respective parts of the transect. It was soon evident that it was going to be a good count. When I arrived at the end of my section in the so-called Whisky corrie (where there was apparently an illicit still in days gone by) I was delighted to receive a text from the others saying they had seen 48 pearl-bordered fritillaries on their stretch so far!
The yearly counts of pearl-bordered fritillaries have varied from a previous high of 83 in 2011 to a low in 2013 of only 7, so the count this year of 132 is a record! It was very satisfying to have made the right decision about when to do the survey and seeing how these delightful butterflies make the most of short periods of good weather.