Home > Conservation > Butterfly transects in the UTB area, 2007

Butterfly Transects in the Upper Thames Area

Report for 2007 by Mike Wilkins
UTB Transect Coordinator

The results presented here are extracted from transect data from 47 sites throughout the three counties entered into the Transect Walker programme and analysed using Transect Coordinator software. The transects are organised by many different individuals and organisations including Butterfly Conservation and, especially, BBOWT. Most are walked by several people so it can be seen that a large number of individuals are involved. Consequently, we have a considerable amount of data to draw on.

After a wet March the dry, warm April brought out many butterflies about two weeks earlier than 2006. In general, the early species benefited but the very wet May and wet and cool June cut them short to give them a similar flight period. Those emerging in early summer onwards were less early or had a similar flight season to 2006 but their peak counts were reduced. As might have been expected the peak counts for all those species which coincided with the July downpour and flooding, were down to about half those of the exceptional season of 2006. Subsequent counts tailed off much more quickly.

As 2006 counts were anomalously high I have usually avoided direct comparisons with 2007. However, indices for most sites were generally low and of those with three or more years recording, fifteen out of 44 had their worst season yet. However, some compare favourably with previous years, especially Finemere Wood (second best of nine years), Howbery Park (7th of 13), M40 Compensation Area (6th of 17), Aston Upthorpe Downs (3rd of 15), Bowdown (11th of 21) and Hartslock (3rd of 5). I can see no common factor in this selection of sites.

There have only been enough sites for collated indices to be produced for a maximum of the past six years although some sites have been monitored for ten or more years. Only the commoner species have enough sites and counts to produce indices at all.

The all-species flight period in 2007 matches very closely with the aggregate for all years, with a peak in the week beginning 15 July (week 16). However, compared with the previous few years, although the rapid increase in numbers at the end of June was similar, the tailing off after the peak was one to two weeks earlier.

Small Skipper and Essex Skipper are not recorded as individual species on most sites, only as an aggregate. 2007 was the poorest year of the last six for the combined species but only marginally, and the populations seem to be stable. Conversely, Large Skipper shows a significant steady decline over the same period, as calculated from ten to fifteen sites. The flight period was about average but slightly earlier and longer than in 2006. The small amount of data on Dingy Skipper means that no indices were produced. However, total counts were well up in 2007 and the highest for several, though not all sites. It had an average flight period but was up to two weeks earlier than 2006. The same remarks can be applied to the Grizzled Skipper except that it was earlier than most of the last few years.

Small Skipper Photo © Dave Wilton

Green-veined White Photo © David Redhead

The Brimstone index fell slightly from 2005/6 but there is a definite upward trend over the last six years. Large White had by far its poorest year and Green-veined White its second lowest index (2006 was lower) giving a downward trend over the same period but with both spring and summer peaks 1-2 weeks earlier than recent years. The index for Small White was also the lowest but with no significant trend. Orange-tip has declined over the last three years to produce the second lowest of five but with little variation. 2007 was an early year but it had a very short season.

Brown Argus Photo © Allen Beechey

There was only a small drop in the index for Small Copper in 2007 which had an average year. It was another early species for which both broods peaked early in 2007 but unlike most recent years there was no build up to a late summer peak. It had tailed off by mid-September. Brown Argus suffered a sharp drop in 2007 after a steady climb from 2002 but the calculated trend is not significant. This was in spite of the second brood unusually being smaller than the first. Common Blue peaked in 2003 but 2006 was not far short. Although it suffered a drastic decline in 2007 to fall well below the previous lowest in 2002 there is no significant overall trend. Again, it was the second brood which was particularly poor. The sharp fall in the index for Chalkhill Blue followed a steady climb from 2004-6 resulting in an overall decline. The flight season was average, as was the case with most summer species. The Holly Blue, which has a cycle of population crashes had a better year in 2007 than 2006 to produce an average year for the last five. We have insufficient data to make any observations on the Silver-studded Blue.

There are insufficient counts to produce indices for White Admiral on the four sites which record it regularly. Counts were well down on the good years of 2005/6, in an average flight period.

The population of the mainly immigrant Red Admiral is dependent on conditions on the Continent which determines how many arrive here and breed to produce a local brood. The peak year was in 2006 but there was a slight fall in 2007, bringing it below 2003 as well. A slight upward trend is not significant. The much more erratic immigrant, the Painted Lady had a peak in 2003, declining thereafter, with no indices for 2005/6. An index was calculated again for 2007 but well below those for 2002-4. We await another 'Painted Lady' year.

As most people would guess the Small Tortoiseshell has been declining sharply from a peak in 2003. However, there was a slight recovery in 2007. It was also the worst year for Comma after a slight but steady increase over the previous years. In contrast the Peacock had its best year of the six, beating the previous best year, 2003, by a good margin.

The Dark Green Fritillary is only recorded regularly and in double figures at Swyncombe Down. The peak in the six years of recording was in 2006 but 2007 came second, just above 2004/5. Only at Bowdown has Silver-washed Fritillary been recorded for a long time and 2007 was its second best year, behind an exceptional peak in 2006. It seems to be increasing at Dancers End and Moor Copse over the shorter period that they have been running and several new sites recorded it in 2007 so we may be able to say more about it in a few years time.

The Speckled Wood has shown a significant steady decline during the period with 2007 by far the lowest, although it made an early start. It was also the worst year for the Gatekeeper which has declined from a peak in 2004. It had an average flight period, unlike the early season in 2006.

Although it emerged about a week earlier in 2007 than the previous few years Marbled White peaked in 2006 and there was a sharp drop in counts in 2007. However, it was still higher than 2002, and shows no overall trend. The pattern for the Meadow Brown is almost identical. After a peak in 2003 Ringlets have been steady at a lower level with the last four years virtually equal. Grayling data are too sparse for any conclusions to be made.

The Small Heath rose to a maximum in 2005 and has declined since to drop to its lowest of the six years although there is no significant overall trend. A large part of the decline in 2007 can be attributed to an almost complete absence of an early September emergence.

I should like to thank all those who have walked transects over many years and the organisers of the individual transects for entering and forwarding their data so promptly. Special thanks to Debbie Lewis for supplying BBOWT transect data for a high proportion of the sites used in these analyses. Ian Middlebrook of UKBMS gave considerable support with the new software.