Thursday, 22 June 2017

Cause and effect …

Purple Emperor (Apatura iris)

2017 is turning out to be a very early season for many species of butterfly. My first sighting of iris this year came on Saturday, 17th June when I recorded three males and a female in Chiddingfold Forest. On the same day, a friend recorded four males in a nearby area of the same forestry complex. Our joint feelings were that they’d been out for a maximum of 48 hours. The earliest I’ve ever recorded iris is a sighting of a solitary individual on 12th June 2015. However, for whatever reason(s), that same year I didn’t see another in my local woodlands until 4th July. A highly unusual and freak encounter or are things changing?

One must always be cautious when claiming ‘cause and effect’ with regards to the timing of a butterfly’s flight season. One thing that is evident is that the vast majority of our butterfly species have demonstrated a clear and unequivocal response to climate warming, with their average first appearance dates and abundance peaks now being significantly earlier than they were (Blencowe and Hulme, 2017). The flight periods of most of our species, measured in terms of first annual appearance, the first appearance of subsequent broods (in polyvoltine species), and abundance peaks, have moved forward by between one and three weeks in 20 years, with an average based on 40 species being about 13 days. Interestingly, a few species registered very little response to a warming climate; these included Meadow Brown, Gatekeeper, Silver-washed Fritillary and Brown Hairstreak (Blencowe and Hulme, 2017).

These days iris usually flies from late June to early August, with most adults emerging over a three week period; the average flight season recorded between 2010 and 2014 in Sussex was 28th June to 9th August. However, during this five-year period, two late years (by modern standards) were recorded. The typical first appearance date during the 21st century is now a little earlier, usually falling between the 20th and 25th June. During the later decades of the last century iris usually emerged in early July, so the flight period has shifted forward significantly, by at least two weeks (Blencowe and Hulme, 2017).

The above images show (i) a male from 17th June 2017, (ii) a male from 23rd June 2017, (iii) a female from 19th June 2017, and (iv, v, vi and vii) four images of two females from 21st June 2017.

More at:

Blencowe, M. and Hulme, N. (2017). The Butterflies of Sussex. Newbury, Berkshire: Pisces Publications on behalf of Butterfly Conservation (Sussex Branch), pp. 27, 170-179.

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