Survey Tips - how to find adult White-letter Hairstreaks

Andrew Middleton and Liz Goodyear have been looking for Purple Emperor for several years, and this work has focused their attention on assessing aboreal habitats and their associated species. They have found that many of the techniques that are so useful in surveying for Purple Emperor can also be used to find White-letter Hairstreak, and these tips reflect that experience........

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	Survey tips and recording notes index page

HOW TO FIND ADULT WHITE-LETTER HAIRSTREAKS

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	Watching and timing
	Identification
	Habitat
	Honeydew and nectar

Watching and timing
Books tend to still say that White-letters fly from mid July, but this is best ignored. Yes, this might be the most likely time to encounter a nectaring butterfly but not active males. Many books have said that Purple Emperors are best seen in August; our experience is that by August nearly all activity has finished. This is almost the exact same situation that applies to White-letter. Global warming may be having an effect, although this doesn't seem to show up in the UK BMS transect data, and if so, would be bringing emergence dates forward (which might in turn lead to a smaller window for bird predation for both Purple Emperor and White-letter). Transect data shows records peaking beyond mid-July, but this undoubtedly relates to nectaring adults rather the active arboreal males.

We like to look from mid-June (a convenient time when Grizzled Skippers have almost finished flying and Purple Emperor have yet to emerge!), and we suspect were one to try earlier in the month, one could well have success in the south of the UK. This is when the male activity is at its peak around the tree-tops, with regular chasing groups and twinned males spiralling high into the sky. In our opinion it's the easiest time to find colonies by looking for adults. We have had most success as the morning warms up and conditions become amenable for active flight, whilst activity seems to slow during the middle of the day. Changeable weather can also be good, as the appearance of the sun after cloud invariably triggers the butterfly to take a spin around its canopy. We've also enjoyed watching strong White-letter activity on warm evenings, but our experience is that the light can make observation harder. Its very important to have a good view of all the trees and late afternoon/evening sunshine can make this more difficult. An example of the early morning activity was noted by Liz at a site in south Hertfordshire which she was monitoring in 2000. It was a large site and elm would first be walked past at around 10 in the morning and White-letter would immediately be observed flying in the tops of elm. Two hours later Liz would walk past again and on nearly every occasion nothing would be seen.

This chart compares the timing of recent sightings by Liz Goodyear and Andrew Middleton, with the Butterfly Conservation UK Butterfly Monitoring Scheme database over two time periods and all Herts & Middx records for 2000-4 (The transect at Northward Hill generated much UK BMS data in the first half of July, however we are researching the extent to which static observations of suitable canopy may have been used here)

Identification
Make sure you take good binoculars but don't worry too much - certainly not in the initial two week window at the end of June/beginning of July ~ small dark triangular butterflies jittering in classic hairstreak style around elm-rich spots in late June are almost certainly going to be White-letter. Just beware the rare black hairstreak restricted to a few small areas of east-central England (Butterfly Conservation Black Hairstreak species information). If you aren't too sure, be cautious at first, but once you have made a few positive identifications, hopefully you will find that the species may be more common than initially expected. Whilst White-letter and Purple Hairstreak remain fresh and active, they can be separated fairly easily on size and colour with a little experience, White-letter being smaller and darker, and Purple being larger and giving a paler silvery impression in flight.  Another observation that has been mentioned to us is that whereas Purple Hairstreak tend to stay quite close to the tree in flights, White-letter will be seen clashing high into the air, in a similar but manner to Purple Emperor. As the season progresses, be more cautious, as in addition to Purple Hairstreak, Vapourer moths also start to appear (many Vapourer moths have also been confused with Brown Hairstreak!). By mid July, a perched or nectaring White-letter really needs to be seen well, as activity by now in the tree-tops will be considerably reduced and just one flight can't be considered sufficient for a positive identification. Even Small/Essex Skipper were seen mimicking White-letter by flying up and over elm!

The key to finding the White-letter flights is to imagine you are the butterfly and where would you be? It's vital to get an all round view of the elm, which is one reason why the smaller quantities are easier to observe. Look at all the elm and adjacent trees - is there ash or field maple? If so, then look at these as well. What direction is the sun shining and where is the most sheltered area of the trees and is this in the sun? The key is to watch the sunniest most sheltered side of the trees, walk around and keep looking. Do try and go out in pairs - it's much easier to see activity if there are two people looking, especially if you trying to watch a large area (e.g. Marshland Wood). If the weather is very hot (as was the case in 2006) then it's possible the butterfly will be flying in the shade, which was the case near Elveden in Suffolk.

Habitat
Almost all the elm we have found in Hertfordshire & Middlesex holds a colony, right down to unpromising single trees beside busy main roads in industrial areas. The species appears to be quite mobile and well able to disperse, as we have seen it several times flying well away from elm and it seems perfectly able to colonise quite isolated lone elms. However, there are four healthy elms planted at two sites in Enfield which are probably non-native trees. They seem to drop their seed discs quite early and White-letter have not been recorded on them.

Honeydew and nectar
White-letter, like Purple Hairstreaks, can often be found high up in ash trees and on field maples, and no doubt there are other trees they use. They love nectaring on flowering lime, as do bees, and it's essential to watch any other trees in the vicinity of elm. During the summer of 2006, in addition to elm, we observed White-letter flights from ash, field maple, oak, sycamore and horse chestnut. Whilst White-letter does come down in June to flowers such as bramble and thistle, it tends to be mid-July into early August when this really gets going. However, many of the sites we know don't have any adjacent herbaceous nectar sources, and it may not always be such a reliable way of recording the species.

Liz Goodyear and Andrew Middleton
January 2007