If you’ve ever wondered how people carry out butterfly surveys, you might have imagined ecologists strolling through leafy nature reserves on a sunny day counting butterflies nectaring on wildflowers.

While this is a perfect method for monitoring many butterflies, there are species which spend so much of their time high up in the trees that we just can’t count them.

One of these species is the beautiful brown hairstreak (Thecla betulae) - a small but charming butterfly with bright orange underwings.

In summer, the adults congregate around the tops of trees, especially ash, where they mate and feed on a sticky, sugary nectar secreted by aphids called honeydew.

Normally when we survey butterflies, we walk specific routes around our nature reserves imagining a five metre square box in front of us as we go. Any butterflies that are in that box as we’re walking are counted.

The top of that box, however, is well below the tops of our ash trees.

To monitor brown hairstreaks, therefore, we use a very different method which, unfortunately, does not involve sunny weather or wildflowers.

Instead, we painstakingly count the tiny, pinhead-sized white eggs which the adults lay individually or in pairs on the spiny twigs of blackthorn bushes.

Brown hairstreaks survive over winter in these miniscule eggs before tiny caterpillars emerge in spring to munch the fresh blackthorn leaves.

The best time to find the eggs, therefore, is from December to February, when the blackthorn bushes are bare and, if you’re lucky, the bright white eggs will stand out against the dark twigs and thorns, looking like mini sea-urchin shells with their dimpled surfaces.

In some years it’s been so cold that we’ve had to postpone counts because of thick frost coating the hedgerows and obscuring the eggs.

But this year, in mid-January, a group of BBOWT wildlife trainees and volunteers headed out on a cold but bright day for the annual count at two of our nature reserves between Oxford and Bicester: our Leaches Farm site and our Ludgershall Meadows site which is actually our newest nature reserve, bought in 2022 after a fantastic £330,000 public fundraising appeal.

At each reserve, we divided the hedgerows into 15-metre sections then shared these sections between the group.

Our intrepid volunteers and trainees then painstakingly examined every single blackthorn twig to count the tiny white eggs, taking care to avoid thorns in eyes along the way!

Luckily we had a particularly large group surveying this year, because there was over 1.5km of hedgerow to search through – a huge task if there are only a few people.  

The hard work paid off: this year we counted a record 201 eggs at Leaches Farm (up from just 39 at the last count) and 18 at Ludgershall.

With some of the trainees joining their first ever egg count, there were plenty of opportunities to use hand lenses to see the delicate eggs and examine their patterned surfaces.

Brown hairstreaks aren’t the only species that lay their eggs in blackthorn, and we also found eggs of several others: lackey moth eggs were spotted in tight spirals round twigs, as were two different eggs from unidentified moth species, one pair looking like miniature pink bars of soap.

We were also delighted to find a single egg laid by a black hairstreak ­– a butterfly even rarer than the brown hairstreak.

This species lays on older blackthorn twigs and the eggs are normally found slightly higher in the hedgerow. They are a beigey brown colour, providing better camouflage and making them much harder to spot.  

Brown hairstreaks have declined significantly in the UK, and they are now only found in about seven areas in southern England and Wales.

At BBOWT, we specifically manage hedgerows and scrub at our nature reserves to benefit brown hairstreaks. It is therefore essential to monitor their numbers to ensure that our work is having the right effect.

The excellent increase in the number of eggs we found this year might be due to natural variations in population levels, but it is likely our hedgerow management is contributing to their success.

Numbers of this butterfly are falling due to, among other things, yearly hedgerow flailing. This practice, often only to keep hedgerows looking ‘neat’, completely removes the young blackthorn growth, exactly where the eggs are laid, destroying entire populations.

At our reserves we leave hedgerows for at least three years before we trim them and they’re cut in rotation so we never cut all the hedges on one reserve in the same year.

Cutting the hedges encourages the new young blackthorn growth the butterflies lay their eggs on, but the cutting rotation ensures some of this growth is always present and eggs aren’t destroyed every year.

Following this fantastic success, we will now be on the lookout for the adult butterflies in our summer surveys. Hopefully, the record number of eggs translates to an increased adult population, and some of them might even fly low enough for us to spot them!

We might even see some returning to the same hedgerows to lay their eggs, ready for us to count next winter.