Brown Hairstreak (Photo by Rob King)
Hedgerows for hairstreaks
You have a chance of seeing more than half of all the 58 species of British butterfly here at RSPB Pulborough Brooks (although not all in one visit!). The number of butterfly species found on the nature reserve is a reflection on the variety of habitats and in particular the range of plants that are present on site.
Caterpillars often have very specific foodplants; Purple Hairstreaks depend on Oaks, Silver-washed Fritillaries on Violets. The adults will also need a supply of nectar and as different species will be ‘on the wing’ at different times of year, it is essential to provide nectar-rich flowers over a long season – Cuckooflower, Bluebells and Blackthorn blossom in early spring to Fleabane and Ragwort later in the year. In autumn, Comma butterflies find the energy they need from Blackberries.
The most specific habitat management we do is for one particular species; the Brown Hairstreak - a butterfly that has undergone substantial decline in the UK. It’s a fussy butterfly that requires Blackthorn hedgerow and mature Ash trees in close proximity. The female lays her eggs on Blackthorn, but not just any Blackthorn, she prefers the twigs where new growth meets old.
Female egg-laying (Photo by Anna Allum)
We manage the Blackthorn hedgerows on rotation to ensure there is always somewhere suitable for egg-laying. Cutting the hedgerows and scrub back encourages healthy regrowth that’s perfect for Brown Hairstreaks in the following few years and then the denser hedgerows and thickets develop which provide nesting opportunities for birds and a fantastic home for all sorts of wildlife.
Once the eggs hatch in April/May the well camouflaged caterpillars feed on the Blackthorn, resting beneath the leaves in the daytime. They then pupate before emerging as the adult butterflies in late July or August. As only one or two adults emerge for every 200 metres of hedgerow, it could be rather tricky for them to find a mate, but the hairstreaks have an answer to this in the form of a master Ash tree, a bit like a disco for butterflies! The males congregate in the canopy of a favoured large Ash tree to feed on Aphid honeydew and to await the females.
The alternative egg hunt
It may seem strange to be writing about butterflies during the winter months, but now is the best time to check on how the Brown Hairstreak butterfly is faring. We do this by searching for and counting their eggs - the most reliable way of monitoring the population of this elusive butterfly. The technique involves close inspection of the spiky Blackthorn scrub in search of pinhead-sized eggs. Close-up these eggs have a remarkable structure and resemble tiny sea urchins!
Brown Hairstreak egg (Photo by Gary Trew)
Having trained our volunteers to ‘think like a butterfly’ we set off in teams to search for eggs…
Egg hunting
Whilst this wasn’t a ‘record’ year we did find that the numbers have bounced back a bit after a couple of quiet years. We were delighted to find good numbers of eggs in the areas where we had targeted our habitat management work in recent winters.
In the winter of 2022 we created a good-sized scallop along the trailside in ‘Adder Alley’ and the vegetation has responded beautifully. Last spring the floor was carpeted with Primroses and into the summer months, there was a lovely display on Red Campion – just a bit more light and warmth stimulated lots of wildflower growth. There is also plenty of Blackthorn regrowth in this scallop.
One of our volunteers saw two female Brown Hairstreaks in this area in the summer so it was very satisfying to find that eggs had been laid.
Female Brown Hairstreak (Photo by Rob King)
A big thank you to all our volunteers who helped search for eggs on our alternative egg hunt this year.
Brilliant butterflies at Pulborough Brooks
If you’re keen to see butterflies at the nature reserve then there are a few species that can be seen even at this time of year. On the brighter, sunnier days look out for Red Admiral, Comma, Small Tortoiseshell, Peacock and Brimstone.
Brimstone (Photo by Anne Harwood)