Brown Hairstreak survey - Herts & Middx Butterfly Conservation

Brown Hairstreak and the Blackthorn (Sloe) Survey

The first Brown Hairstreak (Thecla betulae) eggs in Middlesex were found at the end of 2016, in a park just north of the River Thames in the London Borough [LB] of Richmond. However, it soon became apparent that this wasn't just a one-off find with more eggs being found at a nearby site in 2017 (LB of Hounslow). However, the highlight came in September 2018 when the branch had a report (with photo) of an adult Brown Hairstreak in an Ickenham garden (LB of Hillingdon), which lead the branch to discover a previously unknown but thriving colony at Stafford Road Open Space, South Ruislip. The hedgerows and scrub belt are blackthorn (Prunus spinos) rich and winter egg searches have found numerous eggs with adults being reported in the summer. In the summer of 2023, multiple adults were observed and a branch egg survey at the beginning of 2024 found over 100 eggs in the scrub belt! Since the early finds, eggs have been found at many new sites, many are managed open green spaces with blackthorn rich hedgerows!

A single egg was found at Totteridge Fields in the LB of Barnet close to the M25 corridor in January 2021 but by the autumn of 2023, this number had increased to 14 eggs and only a small area of the site was searched and a further search in February 2024 recorded 40 eggs scattered across the entire site. The butterfly is now well established at Horsenden Hill (LB of Ealing), Fryent Country Park (Brent Council) and in January 2024 eggs have been found at Stanmore Common (LB of Harrow) only a short distance from the Hertfordshire border. Finally, after a lot of searching Brown Hairstreak eggs were recorded at several sites in south Hertfordshire - including Merry Hill near Bushey, Croxley Common Moor and South Oxhey Playing Fields!

There are also quite regular reports from River Thames corridor especially in the Sunbury area (which is in the Borough of Spelthorne but still part of the Middlesex branch area). The Brown Hairstreak is not just confined to the outer London Boroughs, eggs and adults have been seen in one of the inner London Boroughs at Wormwood Scrubs and Margravine Cemetery (both LB of Hammersmith & Fulham).

There are several theories behind the discovery of all these previously unknown sites. What is acknowledged is that colonies of Brown Hairstreak in south London and north Surrey have expanded northwards. What is also thought is that they are using the river corridors so that the River Crane and River Pinn/Yeading Brook are being favoured and acting as conduits but not in every case. The 2024 egg finds in Hertfordshire suggest that the River Colne corridor is also a conduit.

However, whether the butterfly is expanding from Ickenham southwards as well as northwards also can't be discounted. In addition since 2022 there have been reports from several parts of the Greater London area of historic Essex so there is also a possible east-west movement which mustn't be ignored either and then you have the question 'why is the butterfly doing so well in the London area?' The majority of sites found have blackthorn and or prunus species but not necessarily in large quantities but they do seem to prefer sites with plenty of Ash (Fraxinus excelsior) (although with ash dieback many ashes are now being lost). The males use the ash as a master tree - somewhere to gather, meet up with females and feed on aphid honeydew. The geology is also relevant but this gets more complicated but will probably explain why one site is thriving but a large swathe of blackthorn/prunus a short distance away is ignored.

The adult is very elusive and hard to find - the colony found at Ickenham is an illustration of this because there was an historic report from the 1983 of a Brown Hairstreak close to the A40, but the butterfly was only rediscovered in 2018 but already present in good numbers! We have also heard of a sighting in 1987 of an adult seen just south of Cranford Park! The eggs although very small are much easier to find and like other Butterfly Conservation branches most sites are found through egg hunting and not relying on adult sightings. The eggs become even easier to find once the leaves have dropped off the blackthorn or prunus species but before they start to flower. Trees covered in lichen are best avoided and relatively new growth is preferred. Much literature discusses suckering growth but also younger stems on uncut hedges with no suckering growth can be searched with equal success in our area. Remember like many butterflies they do not always read the books!

blackthorn 3 Mar 2020

One site was even found under the flight path of the north runway of Heathrow Airport.

Since 2017 a lot of sites have been visited and a Google Map is linked to show where the butterfly or its eggs have been recorded and where we know there is blackthorn and/or prunus. However, there are still many places waiting to be visited. Maybe just a footpath by a railway line or stream-side path might just hold a few bushes but could have been found by the butterfly. Many of our sites are just small areas of blackthorn tucked away along a footpath or on the edge of a recreational area.

Key to markers:

  • Brown Hairstreak (eggs & adults) have been recorded at this site in Middlesex, and/or a neighbouring branch (various coloured ticks are now used denoting the year!)
  • Blackthorn has been noted here and in most cases searched for eggs (a round green circle indicates that a site previously searched has now been colonised!)
  • This area has been visited and either no blackthorn or very little found
  • An area considered unlikely to support Brown Hairstreak and has been visited (note some of these sites were visited several years ago and in one recent case found now to support Brown Hairstreak eggs!)
  • Areas that might have suitable habitat and needs visiting and surveyed for blackthorn/Brown Hairstreak

It is known that Brown Hairstreak will lay their eggs on a variety of prunus species but blackthorn is the recognised favourite. However, when confronted with a hedge line of prunus it isn't always that easy to tell them apart. Identifying prunus species? That's really hard unless you are a tree/flower expert! One key factor with regard to Cherry Plum (Prunus cerasiferais) - is that it flowers earlier than blackthorn. For an example it was already starting to flower at the beginning of March in 2020, whereas blackthorn was nearly a month later. An additional reminder, despite its thorns, hawthorn Crataegus monogyna is not in the same family and not used by Brown Hairstreak.

blackthorn 29 Dec 2018 blackthorn 5 Jan 2019 blackthorn 24 Jan 2021

Above are a few examples of a fairly typical blackthorn or prunus hedgerow that you might encounter on a local walk.

The YouTube video on finding Brown Hairstreak eggs presented on 1 December 2021 should be watched before looking for eggs - click on the image below

Finding Brown Hairstreak eggs

Please be aware that several species of moth also lay eggs on blackthorn and these are shown in the video as well. BC Upper Thames Branch has published a very helpful guide to the eggs found on prunus at this link on Twitter. Because there will be other eggs on the blackthorn or prunus do remember to take a hand lens. The moth egg records are also valuable so do report them to the Herts & Middlesex Moth Group.

A YouTube video from BC Surrey & SW London Branch is very informative and gives lots of additional information, on looking for blackthorn and finding eggs of Brown Hairstreak plus the occasional moth egg! Click on this link to view.Some of the sites in Surrey are a slightly different habitat type especially in the more rural settings so don't compare everything.

So please report back what blackthorn or prunus you can find, ideally supported with a photo to give us an idea of extent and quality. Please also provide either an Ordnance Survey grid reference; postcode or what3words location and send an email to Liz Goodyear ( Having found new areas, why not then return next summer to record your local butterflies? Just remember you could enjoy the satisfaction of finding a new site for Brown Hairstreak!