Surveying Butterflies – A Report from the Field

From 1st April to 30th September every year, volunteers all over the country conduct a Butterfly Transect study on a weekly basis. What is that you may well ask?  Well, in simple terms, its a weekly recording of butterfly numbers and species within a defined area, which has to be repeated exactly.  The records produced are sent to the United Kingdom Butterfly Monitoring Scheme (UKBMS), from which numbers and trends in number of UK butterflies can be analysed on a national basis.

Holly blue – one of the species recorded in April

To assist in that locally, I have been doing a transect in the Bovey Valley in the vicinity of Hisley and Pullabrook Woods for the last 5 years.  If you are not familiar with the transect method, this is described by the UKBMS, “A transect is fixed-route walk on which butterflies are recorded along the route on a regular basis (weekly) under reasonable weather conditions…. Transect routes are chosen to sample evenly the habitat types and management activity on sites… Transects are typically about 2-4km long, taking between 45 minutes and two hours to walk.” As the transect I walk is situated in the East Dartmoor National Nature Reserve, that information feeds into other local transects to help compile a picture of any changes in numbers in the immediate area.

The start of APRIL this year was relatively cool and for the first two weeks of the month there was very little activity.  However, the middle of the month saw a large increase in temperature and the amount of sunshine and there was a sudden and large increase in butterfly numbers.  There were no great surprises in which butterflies were seen for springtime, in particular I recorded brimstone, peacock, holly blue, speckled wood and good numbers of orange tip. 

Good numbers of orange-tip were recorded in April

Moving from April into MAY, I was particularly looking out for pearl-bordered fritillary and then small pearl-bordered fritillary. I wasn’t disappointed. In particular the warm weather towards the end of the month made them more prevalent and I recorded them in all the usual places I expect to find them.  This is because they are “pesky little blighters” and are very limited geographically due to the requirements of their habitat. They like south facing slopes with plenty of bracken litter and dog violets aplenty. The aspect means they get warmth, they chrysalis over winter in the bracken litter and caterpillars feed on the violets. So they have very specific requirements!  Despite all that, encouragingly they exist in good numbers in the NNR.

The other thing about pearl-bordered fritillary and small pearl-bordered fritillary is that they are very difficult to tell apart with the naked eye. The differences in colouring, markings and size are not that large and personally I have to study them hard and individually to make them out. Usually I do this by taking good quality photographs and looking at them closely at home. In addition to pearl-bordered fritillary, I have also recorded orange tip, brimstone, peacock, holly blue and the emergence of small heath.

Well JUNE was a bit of a mixed month in terms of weather and correspondingly butterfly numbers varied hugely from day to day.  The best days to look for and record butterflies are warm days, above 17 ° C, and with plenty of sunshine.  The warmth stirs them into action and they love to bask in direct sunlight and open up their wings which makes them easier to find. Continuing on from May, I was very focussed on pearl-bordered and small pearl-bordered fritillaries which can be found in a very “bespoke” habitat. In my experience numbers were not so high this year however, I spotted small pearl-bordered late in June, which is quite unusual.  Clearly their emergence was delayed by the unseasonably cool weather in mid June.

The end of June and beginning of JULY brought much more summer-like weather and the levels of butterfly activity increased exponentially.  That was timely, as on the 8th July I ran a guided walk recording species and numbers in Pullabrook Woods, which enabled me to help people improve their butterfly identification.  It was a successful day with no major surprises but it was heartening to see pretty much what I expected to see at this time of year.  This included: silver washed fritillary, white admiral, red admiral,  brimstone, small heath, meadow brown, ringlet, gatekeeper, comma, painted lady and large skipper.

Written by Simon Smith, Natural England volunteer

If you would like to challenge your butterfly ID skills – now is a good time to look out for the white-letter hairstreak butterfly. Butterfly Conservation are interested in receiving records of any sightings of this distinctive butterfly – click here to download the Finder’s Guide – White Letter-Hairstreak Finder’s Guide

You can learn more about the Yarner Wood transect in this blog written by Natural England volunteer, Janet. https://eastdartmoorwoods.org/2018/07/13/yarners-butterfly-transect/

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