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........
Link sections on this page . . .
HOW TO FIND ADULT WHITE-LETTER HAIRSTREAKS
Link sections on this page . . .
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.
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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)
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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
