by Ellen Lee, Species Data Manager
For me, early winter means that it’s time to scour blackthorn bushes in search of the tiny eggs of the Brown Hairstreak butterfly. In a strange way, I find it relaxing, standing still and just letting my eyes scan over the many bare purple/grey stems of the new growth. I’ve been counting eggs on the Otmoor RSPB reserve for a number of years, but recently it’s also become a regular early winter ritual to do so in my local urban reserve (and Local Wildlife Site) Milham Ford Nature Park in Oxford too.
Adult Brown Hairstreaks are cryptically coloured and not given to advertising their presence by flying around in the open. So, we see very few adults, even on sites where we know they are present in decent numbers. Thank goodness that they lay white eggs on bare branches of new growth. Despite their tiny size (0.7mm diameter allegedly) the eggs are fairly easy to see, once you’ve got your eye in. It’s the count of eggs that we use to monitor the relative size of a population. Judging the absolute population size is much more difficult because how many eggs females usually lay isn’t known and anyway may vary between individuals and from year to year.
Brown Hairstreak was first reported at Milham Ford in 2008, but it was in 2018 that the first six eggs were found. I started my surveys in the winter of 2020/21 when I found a single egg! After that, the numbers grew steadily till in 2023/24 they exceeded 20 for the first time. However, they were very much localized in two areas, the northern hedge and a single cherry plum near the ponds. So last winter, I was thrilled to not only find 35 eggs in total, but also to discover that they were in a couple of new areas. However, I wasn’t prepared for the sudden population explosion I encountered this year! My final total for 2025/26 is 116 eggs and I’ve found them everywhere on the reserve where there’s either blackthorn or cherry plum, including on the southern side of the reserve (which is north facing and therefore not supposed to be favoured) and on the external sides of the hedges facing Jack Straw’s Lane and Marston Road.
What an amazing success story! I’ll be interested to find out whether it’s related to Brown Hairstreaks generally having a very good year, or whether it’s more to do with the management at Milham Ford which has tried (as much as is feasible) to go easy on cutting back blackthorn and allowing for some areas for saplings which brown hairstreaks particularly like laying on.
