Monday, 21 August 2006

Visiting fritillary draws the crowds

There have been some very interesting reports over recent weeks of butterflies not often seen in our region. Some of these, like clouded yellow and camberwell beauty, are immigrants from the Continent, while others probably originate from closer to home. One particular individual, a silver-washed fritillary, put in an appearance at Natural Surroundings, near Holt, and attracted quite a crowd when its presence was put out on the pagers used by many Norfolk birdwatchers. (The middle of the summer is often a quiet time bird-wise and, in response, many birdwatchers take up a passing interest in butterflies and dragonflies). Given that this particular species of fritillary effectively became extinct in our region several decades ago, and that it is similar in appearance to the dark green fritillary (which does breed in Norfolk), there was understandable controversy as to whether this was simply a case of mistaken identity. In the event, the fritillary was well-watched by at least some observers capable of distinguishing the two species and its arrival occurred at a time when several other individuals were seen in Cambridgeshire and Suffolk. This species is largely restricted to the south and southwest of Britain and, with a powerful flight, it is considerably more mobile than our other fritillaries. During particularly hot summers individuals may be seen well away from the core range – a pattern matched by the butterfly’s response to climatic trends. The population has been shown to expand during warmer periods and contract during cooler ones.

Although I did not make the trip to see the silver-washed fritillary at Natural Surroundings, I did visit two of our dark green fritillary colonies this year, first at Winterton Dunes and then two weeks later at Holkham. Although a species of downs and grassland over much of its core range, the dark green fritillary seems to do very well on coastal sites, either on dunes or on undercliffs. Here the males spend much of the day on the wing, searching for females, and the patrolling flights are characterised by short bursts of rapid wing beats followed by periods of gliding. The females themselves perch low down in the vegetation and the males probably locate them by scent. Once mated, the female will return to perch within the vegetation, soaking up the sun’s warmth to help her eggs ripen. As such, although these butterflies are easy to spot when on the wing, they are difficult to photograph, except during the early morning and late afternoon when nectaring. A late July or early August visit to Winterton Dunes should guarantee views of dark green fritillaries and may just produce a clouded yellow or even a camberwell beauty.

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