Introduction
This is the first year of “championing” the
Silver-spotted Skipper within the Upper Thames Branch (UTB) of Butterfly
Conservation. This report therefore includes some background material as
well as reporting specifically on the year 2003.
General situation
The Silver-spotted Skipper (SSS) is a
warmth-loving species, and is presently at its northern UK limit in the
Chilterns. All UK colonies occur on chalk grassland in southern and
south-eastern England, mainly on steep south-facing slopes. Formerly its
range extended further west and much further north in England, even
including Yorkshire (Millennium Atlas, Asher et al, 2000). In Europe its
range extends well north into Scandinavia. In England its only larval
foodplant is Sheep’s Fescue (Festuca ovina), but in Europe it will
also eat other grasses.
The main concentration in the UTB region lies in a band from Swyncombe
Downs through Watlington, Pyrton and Shirburn Hills to Bald Hill and
Beacon Hill at Aston Rowant, all with strong populations. Further
southwest there is a weaker colony at Aston Upthorpe Downs, while further
northeast there are scattered colonies in the general area of Princes
Risborough to Wendover. Roger Kemp (2001) reported both adults and eggs
seen in 2000 at Chequers/Grangelands, Yoesden Bamk, Butlers Hangings and
Turville. There are isolated records from Ivinghoe Beacon in 1992 and
1995, although it appears that breeding is not currently taking place
there.
An extremely thorough survey was undertaken in 2000 by researchers from
Leeds University, who have kindly provided their report (Davies et al,
2001) and records. These also include records of visits to a large number
of potentially suitable sites where evidence of SSS (adults and/or eggs)
were not found. Their results, recorded by experts, present a
definitive record of the distribution of SSS in the Chilterns for that
year. The Leeds survey also covered the entire English range of the SSS.
Records for the UTB region for years 1984 to 2002 were provided by Jim
Asher. I have also collated provisional records for 2003 communicated by
members of UTB by email, word-of-mouth, and the UTB “Butterfly Diary”.
Many thanks to all for passing on their sightings. Data from the various
sources are plotted below.
Sightings of
Silver-spotted Skippers in the Chilterns area by UTB members 1984-2002,
intensive survey by Leeds University in 2000, and provisional UTB records
for 2003.
Key: AU = Aston
Upthorpe; H = Hartslock; KB = Killdown Bank; T = Turville; Sw,Wat =
Swyncombe Downs, Watlington Hill; Sh,Py,AR: Shirburn & Pyrton Hills, &
Aston Rowant; PR = Princes Risborough; Wen = Wendover; IB = Ivinghoe
Beacon |
Overall numbers in 2003
It is difficult to assess changes in
numbers from year to year because there are only 217 UTB records of SSS in
total between 1984 and 2004, an average of 10 per year. The number of
records in recent years has varied markedly: 19 records in 1999, 18 in
2000, 9 in 2001 and only 5 in 2002. However, this is more likely a
reflection of changes in recording effort than in actual SSS numbers.
The only local transect to have numbers recorded over several years is at
Aston Upthorpe. The numbers there are:
Year 1993 94 95 96 97 98 99 00 01
02 03
Totals 9 12 8 0 11 3
7 9 1 1 4
The drastic reduction there in later years appears to
be a result of sheep-grazing taking place in July and August coupled with
a large rabbit population, so that the sward is bare with few flowers.
At Bald Hill (Aston Upthorpe) 40 SSS were counted in
40 minutes on a very hot day (9/8/03, 36˚C). By contrast, at
Watlington Hill on the following day (hottest on record in UK, 10/8/03,
37.9˚C at Heathrow) only 11 were seen in 60 minutes. Judging
purely from impressions, I would say that Watlington Hill has seen smaller
numbers in recent years than formerly, perhaps due to over-grazing by
rabbits resulting in a very short bare sward.
Single adults were seen in August 2003 at Hartslock
reserve (twice) and Turville Hill, confirming that populations continue to
survive at these two sites, which are isolated from the main colonies by
several kilometres. There were several reports of SSS from the Great
Kimble – Chequers – Coombe Hill area near Wendover, indicating that these
most northerly colonies are surviving, consolidating, and possibly
extending.
Brief surveys made on single days at Killdown Bank
(SU791856 – 797848) and Shirburn Hill revealed no signs of SSS, although
the latter was on a poor day late in August.
|
At one local site, the numbers of SSS seen
per day throughout the flight period are shown above. The flight period
started about two weeks earlier than in 2002 (about as early as it ever
gets), and possibly ended earlier too. The weather on the days walked was
generally hot (19 to 32 ˚C) and sunny in this exceptional summer. It is
sometimes said that SSS will only fly on sunny days with temperatures of
20 ˚C or more, but 15 were seen on a day with 50% sun at 19 ˚C, and 4 or 5
at 20 ˚C with only 5% sun. A similar flight pattern was recorded at
Watlington in 2000 by Robert Wilson (Leeds).
Eggs
Silver-spotted Skippers can sometimes be found more
easily by looking for their eggs, which are prominent and can be found
between August and the following March. They are often described as
pudding-basin shaped and are pale cream or white. They are laid on the
leaves of their food plant Sheep’s Fescue, or sometimes on adjacent
foliage, usually in or near a small patch of bare earth. A
comprehensive search was not made in 2003, but 12 eggs were found at
Beacon Hill (Aston Rowant) on 9 August.
A curious feature of SSS eggs, remarked by Frohawk,
is that they seem very large compared with the small insect that lays
them. This prompted us to investigate the number of eggs laid by
different species, with special attention to the SSS (Soulsby & Redhead,
2001). We were unable to find a book (among the ones we possessed) that
quoted numbers of eggs for every species, and none of the books gave an
estimate for the SSS. Numbers for species that were quoted were
generally between 30 and 500. To get a rough estimate, I made a
volumetric calculation of the number of eggs that could be fitted into the
abdomen of a female SSS, and arrived at a figure of 50-60. This does not
allow for eggs ripening after the first batch is laid, so a total figure
might be 2 to 3 times that number, or say 100-200 per female.
Evidence of range expansion – contraction
There is evidence, as reported by the Leeds
team (Davies et al), that the population in the Chilterns has been
strengthening over the past 20 years, and that the range has extended
northeast. The provisional records for 2003 did not identify a further
extension beyond the Leeds survey, but it is only three years since that
survey. Nonetheless, the 2003 records did seem to indicate some
consolidation in the Princes Risborough – Wendover area.
Reasons for Concern and Optimism
The best evidence for optimism about the
future of the SSS in the Chilterns comes from the Leeds survey (Davies et
al, 2001). Regarding future developments they said “The most promising
development… is the colonisation of habitat near Princes Risborough…..There
are around 45 ha of habitat in the region, and therefore the real prospect
of the establishment of a large network of populations.” They also
pointed out that there is then a gap of 12km in suitable habitat before
arriving at the large areas of suitable habitat around Ivinghoe Beacon and
Pitstone Hill.
Set against that is the diminishing population at Aston Upthorpe, possibly
due to an unsuitable livestock grazing regime. The string of colonies
from Swyncombe to Aston Upthorpe remains strong, although the Watlington
Hill population, once one of the strongest, is now diminished by
over-grazing by rabbits. Likewise, the population estimated for Swyncombe
Downs in 2003 seems to be smaller than that estimated in 2000. There
seems to be some re-population of more distant sites such as Turville and
Hartslock (unless these were previously overlooked). The Leeds report
showed evidence that SSS can colonise suitable sites up to about 5km from
an existing population in 9 years, and 10km in 18 years, and for
north-facing as well as south-facing sites to have become acceptable.
Thus there seems a good likelihood of further areas of suitable habitat
becoming colonised in future.
Acknowledgements
We are very grateful to Zoe Davies and
Robert Wilson of Leeds University for sending us their report and
data-set, and for their helpful advice and comments.
SSS Champions team
Richard Soulsby
Mike Wilkins
Nick Bowles (exchange of records for SSS and Chalkhill Blues)
Why not join us? We need to survey some of the
remoter sites to find evidence of expansion, especially in the area
between Princes Risborough and Ivinghoe Beacon, and on the dip-slope of
the Chilterns. Any casual observations would be welcomed too.
References
Asher, J., Warren, M., Fox, R.,
Harding, P., Jeffcoate, G. & Jeffcoate, S. (2001). The Millennium Atlas
of Butterflies in Britain
and Ireland. Oxford University
Press, Oxford.
Davies, Z.G., Wilson, R.J., Burton, H.M.C., Ericson, K.K.N., Ewin, P.M.,
Glencross, S.C., Goodhand, A.N., Hanna, S., Holloway, C.T.R., Leaper, R.,
Mellings, J.H. & Thomas, C.D. (2001). The status and conservation of the
silver-spotted skipper Hesperia comma in Britain. Confidential
report to English Nature and Butterfly Conservation. Schoo lof Biology,
University of Leeds.
Kemp, R. (2000). Return of the
Adonis Blue and Silver-spotted Skipper to the Bucks. Chilterns.
Newsletter No. 51, Upper
Thames Branch, Butterfly Conservation.
Redhead, D. (2001). Silver-spotted Skipper egg survey. Newsletter No.
51, Upper Thames Branch,
Butterfly Conservation.
Soulsby, R.L. & Redhead, D. (2001). How many eggs does a butterfly lay?
Newsletter No. 52, Upper
Thames Branch, Butterfly Conservation.
Thomas, J.A. (1983). A quick method for estimating butterfly numbers
during surveys. Biological Conservation, 27, 195-211. |
