Brown Hairstreak on the Wing
in Bucks / Oxon Borders |
| Brown Hairstreak have been on the wing since 27th July. The first
sightings were males seen nectaring on thistle and angelica at two sites. The weather on
the following few days was poor. No further adults were seen until the 2nd August when a group of UTB members had the good fortune to see one male at Asham Meads, on elm! and a further three at Whitecross Green Wood, one at ground level and two high up on ash. Is it the case that the males, like many other butterfly species, are first to emerge? Do they spend their first days supping nectar at ground level and subsequently move to the tree tops as the females start their emergence? UTB members have located a group of young ash trees on which adult Brown Hairstreak were seen on successive days. A maximum of six were seen here on 6th August. A high number of eggs have been located on the adjacent blackthorn hedgerows in recent successive years. Do these individuals spend their entire lives at this location? Would the females disperse further afield even though there is seemingly abundant suitable blackthorn immediately available? A number of sightings of adult Brown Hairstreak were reported from other known sites in the first week in August. The vast majority were seen on ash and included females. No incident of definite pairing behaviour was reported. How long would it be before the females sought out blackthorn on which to oviposit? A female was seen ovipositing at Bernwood on the 11th August. Two eggs were found at another location on the same day. Did this mark a general phase of egg laying across the wider area? There has been no clear evidence to confirm such an occurance. Brown Hairstreak have a long flight season. Is the emergence of adults protracted or do they all take flight in a relatively short period? The females descend from the ash canopy to oviposit on suitable blackthorn. Will the males subsequently be visible on the ash trees? The adult Brown Hairstreak has a reputation as the UK's most elusive butterfly. Many UTB members would certainly support this view! However we are fortunate in that the egg stage is relatively easy to find. This fact has provided us with a useful means of locating sites where the butterfly is active and to subsequently seek out adults during the flight season. The Brown Hairstreak in our area appears to be very stable in terms of its distribution. It has been present over many years in a range of sites in the Bucks / Oxfordshire borders and has not colonised any known new sites in recent years. Experience of the butterfly may be different in other parts of the UK. Reports from Surrey members indicate that the butterfly has been lost to some sites in The Surrey Weald but that it has managed to cross the M25 motorway and colonise new areas further north. Yet in our area the butterfly seems incapable of crossing the A41! The Brown Hairstreak is certainly an enigmatic butterfly that raises more questions than answers. If you can contribute in any way to furthering our knowledge we shall be more than pleased to hear from you! By the second and into the third week of August there was far less evidence of adult
activity in the ash trees. The females were now evidently descending to areas of
blackthorn across a range of sites. However views of individuals ovipositing were not very
common although eggs started to turn up on the foodplants. Eggs still seemed to be at
relatively low density. By the final week in August and into September there was evidence across a range of sites that the female brown Hairstreaks were getting down to their main business i.e. egg laying. Although males were seen occasionally in ash tress they were also sighted on at least one site at ground level in this period. This was exceptional as the vast majority of individuals were females involved in ovipositing on blackthorn. These females were almost always in good condition which was at variance with the males mentioned above which seemed exceptionally 'tatty'. Why should there be such a discrepancy between the sexes? Is it a matter of lifestyle? Are the males much more active whilst on the ash trees? Do they have a shorter lifespan? Eggs were still relatively difficult to locate during this period, even on known sites. Will their abundance increase during the coming weeks? In the first week of September I found adult laying at three separate sites which cover
three diagonal kilometre squares on the OS map. The common thread is a railway line that
is adjacent to all three locations.I also found adults adjacent to an M40 road bridge.
This is within sight of the M40 Compensation Area where Brown Hairstreaks are present. The following is an interesting contribution from Wendy and Mick Campbell to the Brown Hairstreak debate: Hello
Tom, - Females ovipositing - I suspect many people miss this event because it's difficult to see the females as they walk around in the Blackthorn looking for a suitable site and then seem to disappear behind a leaf (even if you saw them going there, as we did on a couple of occasions!). - Reduced sightings of males - surely they emerged first and would therefore disappear before the females, as seems to be likely with other species? - Is there a 'peak' egg-laying period? I would have thought that females emerge over a period of days/weeks and therefore egg-laying would also be staggered. It could also be weather-related. If there had been a prolonged period of bad weather and then the sun came out, I would imagine all the females would start laying their eggs in a hurry (hence creating a 'peak' egg-laying period). This year has been exceptional in terms of the number of sunny days and therefore I would suggest the females have spread their egg-laying over a longer period, with no obvious peak period? And a question which I have: do all eggs hatch at the same time or is the hatching period staggered? I've recently been rearing a small number of moth larvae (from eggs 'left' by a female after a moth trap evening) and although all the eggs (from the one female) hatched the same morning, the larvae developed at different rates. The resulting moths emerged on different days - males emerging first, then females a few days later! I wonder if this is a common pattern for moths and butterflies? Best regards, Wendy & Mick Thanks to David Redhead for the following: Hi Tom, Wendy raises some interesting questions. Some possible answers below. About two dozen Brown Hairstreak eggs were tagged on blackthorn hedges at Otmoor Rifle Range last New Years Day. These were inspected regularly for hatching from the end of March. The first egg hatched between the 2nd & 9th April and no eggs hatched after the 23rd April with about a third of the eggs failing to hatch at all. So hatching occurred over approximately a fortnight. I would expect "captive" eggs to hatch much closer together than "wild" eggs as they will have all been subjected to more or less identical conditions. The "wild" Brown Hairstreak eggs will experience differing micro-habitat conditions dependent on their position on the blackthorn hedge. The first record of egg-laying this year occurred on the 9th August and egg-laying was definitely still in progress during the first week of September. So we obviously have a significantly longer egg-laying window than hatching window. As the eggs over-winter with a "life" of about 8 months I would not expect there to be any relationship between laying and hatching time. What will determine the hatching time are the weather conditions in March/April when the caterpillar is forming within the egg. Interestingly, the books say the eggs hatch from late April to early May - so 2003 was an early season from the start. If anybody is lucky enough to see a female egg-laying please try and observe the egg for the first few minutes of its life, ideally with a hand lens. I think you will find it surprising and equally rewarding as continuing to watch the adult, which is the natural thing to do. I would love to hear from anybody who manages to do this. Is there a peak egg-laying period? I think it is impossible to definitely answer this with so few observations being made of egg-laying. My feeling is that egg-laying occurs over about 6 weeks with a skewed normal distribution i.e. the frequency will increase to a maximum after about 2 weeks and then gradually tail off - but this is pure speculation. David Redhead. Observation: A number of eggs can commonly be seen at one
location on a blackthorn twig. Rob Mabbett from Gloucestershire BC informs us: 'In 1999 at the Roundhill area,I observed a female alight on a twig and do her usual walk down the blackthorn, locate a suitable spot and lay one egg. She then flew to the top of the same twig and repeated the procedure laying the second egg adjacent to the first.' Has this behaviour been seen by others? 13th September: There is evidence that females are continuing to lay. Perhaps this is
no surprise as egg laying is their modus operandi. Final question - why does every question concerning BHS only generate further questions? Counting Eddies is much more straightforward! Dennis Dell reply to above: Interesting! I've only observed ovipositing once, and it was a single lay. Thank you David for the following: David. These notes will be added to as the season
unfolds! |
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