This is a “migrant addition” to my almost complete (bar one) British butterfly list. Large Tortoiseshell is one of a select group to have made inroads, with global warming into southern and south-east England and East Anglia over the last 15 years or so. Hence when I was invited by Ewan to observe a cluster of emerged hibernators from a small breeding colony in a Kent woodland, the opportunity was sufficiently evolved to arouse my interest.

Large Tortoiseshell occurs across continental Europe, being common in some regions though erratic in others. The species was once widespread nationally, mostly in central and southern English woodland habitat, particularly such containing the historic food plant Elm and where blooming Sallows and Cherries provided nectar sources in spring. But its numbers were always known to fluctuate and the butterfly became extinct here by the 1960s. There were various suggested causes for the decline including climate change, parasitism and the impact of Dutch Elm disease. As the species was disappearing in the UK, its numbers also decreased dramatically elsewhere in north-western Europe. 

My pre-trip research revealed that since then there have been sporadic national records annually, mostly in high summer from the south coast and Isle of Wight. It was always open to question whether those might be released by breeders from reared stock rather than genuine migrants. The spring of 2020 brought a marked influx by modern standards when sightings in Norfolk and Suffolk exceeded national totals over the previous 10 years. At the same time other European countries including Holland experienced their best emergences in 40 years. There have also been a few sightings of what are considered to be hibernators each spring since 2018 on the Isle of Portland in Dorset, and breeding has been confirmed there and at Sheringham in Norfolk.

I myself had recorded this butterfly pictorially, just once previously in Sardinia in 2018. Today the weather forecast that prompted our visit was for temperatures as high as 19 degrees, to match conditions earlier this month when my companion had first been alerted to our potential experience. That was certainly warm enough for Brimstones, my first for this season having been on 2nd March locally, and hibernators to be showing themselves. In the event this spring equinox was actually the warmest day of the year so far in southern England.

We arrived on site at 1pm to find a full car park, with butterfly people’s vehicles most likely outnumbering those of dog walkers. It wasn’t until meeting and listening to other observers here that I realised Forestry England Orlestone Forest (TQ 985347) is a further well-known (in informed circles) and established breeding location of some four-years’ standing. But there are thought to be around 12 individuals flying here at present, which to me suggested the colony is quite fragile. The first warm days of spring when hibernators emerge are said to be the most likely time to connect with Large Tortoiseshell, since the single-brooded insects in high summer are only on the wing for a couple of weeks and generally more elusive.

From near the parking area a long ride leads downward and into the woods. When we found and made our way along this we met a group walking up that contained some clearly very seasoned butterfly people, and a high-profile figure who was known to us. Soon someone located a Large Tortoiseshell on the ground and everyone got pictures of it (lead picture, above). We learned that our quest might be encountered anywhere along this ride, but the main concentration of observers was some way further along in a low-lying area that due to its warmth is especially attractive to the butterflies. Several specimens had been present there during the previous day.

Fortunately the gathering today was of genuine butterfly enthusiasts and not peripheral “butterfly tourists” as I term them, suggesting this site is not yet well known to that social-media generated underclass that always creates a circus around scarce species locations. When we reached the spot in question we further learned that as the sun begins to lower from around 2:30pm the LTs would settle open-winged against tree trunks. So this proved twice while we were there, the image below being the best I could manage in the circumstances, given the competition for camera angles. Ewan, through a combination of better position and equipment, fared a little better at this point (see here).

These butterflies were quite like Purple Emperor in their behaviour, starting their day in the tree-tops then coming down to ground level as conditions warm up. They were at all times fast-moving and elusive. But their graceful, gliding flight set them apart from the numbers of Comma and Peacock that were on the wing here, while even more new-season Brimstone were nectaring on scattered forest floor carpets of Primrose and Wood Anenome.

In around three hours here today we encountered possibly five Large Tortoiseshell, while there are reputed to be more in other parts of Orlestone Forest. This is not of course my preferred mode of butterflying, which is alone when it is then possible to gain the confidence of and enjoy communion with individual subjects. I was unlikely to attain pictures that then arise of the standard I seek today, given the constraints of being in a jostling group; ie lack of free movement and not wanting to be the one who puts subjects up. Whenever they did go, LT would always fly high and a long way before settling again. But I would not have gained this conversion had I been alone, so that is the trade-off.