What are butterfly transects?

Simply put butterfly transects are walks where monitors count butterflies.
Farmer Tim Scott saw me doing the Barton route, the latest one on Thursday afternoon and asked the question whether they are counted on random days or if there’s a system. There very much is a system, so for those who don’t know here is a brief explanation:

Butterfly transects are conducted on the same route, every week, starting on April 1st and finishing at the end of September. This year, day one of the transect season was a Wednesday, so each week starts on Wednesday, there are four weeks in April, week five straddles April/May and we are now into week six of the season. By the end, we will have completed 26 weeks of standardised counts on the same route, and of course it is the same route every year, so the data is comparable over the long term.

The times chosen for the surveys are also strictly controlled by criteria that mean that butterflies are likely to be seen; we don’t go out in wind, rain, cold or cloudy conditions. Specifically, surveys cannot start before 10:45 or after 15:45, so the count is made during the warmer parts of the day.

Weather conditions must be sunny, calm and warm. There are two sets of minima to choose between; 13°C and more than 60% sunshine or 17°C and more than 40% sun. Wind must be below force 6 'strong breeze'. Generally, we’ve had much better weather than this for most of the spring, so picking a time has been quite convenient, but week five was rather tricky with quite a bit of cloud, cooler temperatures and stronger winds.

To the results!

Barton route - week five - 2nd May, just nine butterflies seen, nothing of any great note. 

Barton route
- week six - 7th May, with 15 butterflies of nine species, including a small Heath, new for the year, and a speckled wood which have been around for a while, but this was the first logged during an official survey count. There was also a comma still hanging on, like a lot of the over-wintered butterflies this was looking very worn.

Westfield route - week five - 5th May, with 26 butterflies of six species, including the first small coppers of the year.  

Both the small heath and small copper are species which are in decline in the general countryside, and it’s always good to see them surviving through another year. Both species come through the winter as caterpillars, so the spring is a time when they have to finish feeding on the fresh growth, then pupate and develop into adult butterflies. This takes a bit of a while, so they are never as early in the year as species like orange tip which overwinter as pupae, or brimstones which overwinter as adult butterflies ready to fly as soon as it gets warm!

In my old butterfly book, the main flight period of small heath is given as June, so seeing one in early May shows how seasons are advancing with climate change.

Our next anticipated species will be brown argus and common blue as they both winter as small caterpillars and have a feeding frenzy in the spring. A painted lady or clouded yellow could always make a surprise appearance if their migrations bring them our way…

Dr Vince Lea
Head of Wildlife Monitoring 

(Early counts are in previous blog pieces including Warm weather, butterflies, birds and voles!)

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