SOME ADDITIONAL THOUGHTS ON WINTRY GREEN HAIRSTREAKS

By: Stuart & David Wise, Autumn 2003

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        Over a period of several days we monitored a small number of Green Hairstreaks in the Prince of Wales Park in Eldwick, West Yorks. They had emerged, perhaps fatally early, in the first days of April 2003 following a period of bright dry weather unusual for the time of year. However out of the sun it was distinctly cool. Something similar had happened the previous year when the weather was just as bright but rather cooler in the shade. Then the first sighting had been around April 14th some two weeks earlier than this year's emergence. Though the first Green Hairstreak had been seen in late March in Morecambe Bay the two emergence are not strictly comparable as Morecambe Bay is directly subject to the effects of the Gulf Stream. The Prince of Wales Park on the other side of the Pennines is exposed to the prevailing westerlies and its height above sea level scarcely less than the lower slopes of nearby Ilkley Moor.

        The variation in emergence time suggests the newly fledged populations of West York's have yet to settle in. Yet despite visiting other promising sites like Otley Chevin and Oats Royd in Queensbury where the butterfly is present in considerable numbers on the south facing slopes, we failed to see one other Green Hairstreak. We were particularly surprised by its absence on the Chevin where we spent several hours on a cloudless day searching for it on April 8th. Significantly the bilberry was just that little more advanced in the Park than elsewhere, and there were just enough florets for the butterfly to nectar on as a number of queen bees and hover flies were doing. Could the Hairstreak have known this?  As a fully developed butterfly but still encased within a chrysalis, could it pick up and respond to scent traces from its food plant that was only just beginning to flower?

         On a good day we counted at the most three butterflies but on other days our score might drop to just one. Some nights were extremely cold and the temperature in the Park must have dipped well below zero. We feared the butterflies would perish, especially as we never saw them feed during the brief intervals we were able to observe them closely. But happily we were able to establish that these diminutive butterflies were able to withstand sub zero temperatures for at least four nights on the run.

          In fact we came to the conclusion the behaviour of these early bird insects was dictated entirely by their search for warmth. They would rest on the main body of their food plant never once choosing an exposed or taller shoot. And always fairly low down  and to one side where they were sheltered from the wind. Here they could soak up the sun's rays undisturbed to the full. Rather than perch they would alight briefly on the dead stems of last year's bracken or keel over and flatten themselves on the decaying leaf litter, their underside wings tilted at right angles to the sun's rays. When they do this the 'viridian moment' is at its most intense because the sun's rays are polarised. Ordinarily light vibration take place on all possible planes (they fan out) but in plane polarised light vibration is confined to one plane, the plane of polarisation. If angled to the plane of polarisation the same thing happens to leaves and often when resting on bilberry leaves in particular, the Green Hairstreak can be mistaken for some leaves more than others, depending on the angle of hang. A polarised leaf is also a flattened, almost shadow less, leaf and the slightly convex surfaces on either side of the midrib are flattened out. Thus it appears more two-dimensional than it actually is, closely corresponding to the flat wing surface of a resting Hairstreak. This is why a polarised filter is always recommended when taking images of water because it restores depth and reflection to what otherwise could be a featureless surface. 
 
          We cannot say for sure, to return to these days in early April, if the Green Hairstreaks were all males as this would have meant capturing them and examining them for the androconal scent scales. However as far as we could tell their behaviour was not in the least territorial. Survival, in particular the pressing quest for warmth enabling them to fly, had overtaken the need to reproduce. The behaviour of the males was not in the least aggressive. When two butterflies came into contact a brief scuffle could ensue but lasting only for some three seconds as if some half-remembered instinct had been briefly rekindled. But there was a complete absence of aerial combats and prolonged nuptial flights typical of a colony at the height of its emergence. Up to April 9th the Hairstreaks had ceased to fly by 3pm in the afternoon. But on that day we saw our one and only Hairstreak at 3-15pm. By 3-45 it had also disappeared but clearly flight times were beginning to extend into the late afternoon.

          The upper atmosphere above four feet was probably too cold and uninviting for aerial displays. However occasionally one would swoop high into the trees but at no point did any of the insects take up a perching position on any of the leafless branches to await the callers (females) or intruders (males). Only on the last day April 13th when it had become noticeably warmer, did we observe a Green Hairstreak fly up into a tall conifer, settling some 45ft above the ground on a spray of pine needles where it stayed for at least 10 minutes before suddenly taking off. I did not once deviate from looking at the spot until I was satisfied the butterfly had remained there for all that length of time. However this was hardly classic territorial perching behaviour. At that height any territory the butterfly had demarcated as its own would  be an indistinct blur and only the largest objects would be vaguely visible through its compound lenses. The same insect had been repeatedly visiting an area of ground covered in bilberry on which several likely small Larches were beginning to leaf. Not once did it express any interest in them as possible perches even though during the day the number of insects on the wing noticeably increased as the lone, bereft pilots of two weeks standing were joined by the new arrivals. How quickly the colony settled down into a more normal type of behaviour  we cannot say as we left off observing at this point. On this last day we also noticed the butterflies really had begun to shake off their torpor and were far more difficult to approach. One false move and they would instantly be off. This heightened alertness is typical of more normal behaviour and is central to the mating game and staying alive. I recall that last year on the Chevin the few insects that could be seen would alight, now and again, on the lone evergreen conifer at the bottom of 'the big field' but never once on the bare branches of oak and silver birch (see photo). Greenery must be of significance in perching behaviour. Budding oaks as they burst into leaf, display a reddish tinge, the complimentary opposite of the otherwise fresh green leaves. It is thought the red pigmentation to be found on the topsides of new leaves absorbs harmful ultra violet rays that could damage the stomata, essential for photosynthesis, forming on the underside where the bulk of the stomata is to be found. Though quite common in trees and shrubs it is noticeable absent with the Silver Birch which, particularly in northern latitudes, can generally be found in abundance wherever bilberry occurs. Maybe as a quick growing, pioneer, deciduous tree, prolific leaf loss is a necessary fact of life and from early summer onwards the ground around Silver Birch trees is amply flecked with yellowing leaves. Given a choice between a leafing Oak and a Silver Birch it would be instructive to find out if the Green Hairstreak showed a definite preference. In our experience the butterfly will opt for the Oak, a claim retrospectively backed up  by the numerous photos we have of the insect on Oak. 

           The point about this digression is that this reddish hue almost matches the faint pinky brown blush on the lower costal margin of the upper wing when the Hairstreak is in its usual resting or perching position. Seeing the insect habitually rests on the extremity of twigs, this faint, though hardly cryptic, visual echo of its surroundings, may help conceal the butterfly from potential predators.

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            The Green Hairstreak is unique in being Britain's only green butterfly. Green and brown are the choice colours of camouflage and the Green Hairstreak has both, though it never opens its wings to reveal the brown underside when at rest. However the green underside is a clear instance of cryptic adaptation. Even though the butterfly does not resemble a leaf down to virtually the last detail (some other butterflies in fact do) nevertheless it can pass as a leaf to the inattentive observer which crucially will include birds, spiders as well as humans. Unless it specifically catches our eye it remains an idealised representation of a leaf both to us and its predators. This also tells us that the butterfly is not restricted to one particular habitat unlike geometrid caterpillars that closely resemble twigs or Orange Tip pupa that look exactly like a piece of bent stem. That the butterfly abstracts from several backgrounds the better to blend in with them all, is a little studied aspect of crypsis. And yet no sooner have we said this when we are reminded of occasions when the resemblance to a leaf is uncanny (see photo in earlier pamphlet, The Green Hairstreak Colonises the Bradford Metropolitan District 2001, elsewhere on this web)) which only serves to underline the cryptic, almost quantum, nature of this butterfly that appears to flit between two states. We are never quite sure of its actual colour either. Is it green after all? We know the colours of a Red Admiral or Painted Lady but the Green Hairstreaks that we see illustrated or in collections are strangely unfamiliar creatures and different from the insects we see in the wild. Right now I want to see one fluttering in front of me just to check on a few things and yet I doubt, if this were possible, it would clear up the uncertainties I have when recalling, later on, the butterfly to life in my mind's eye.

            Watching these few butterflies that had emerged into a world of frosts, keen spring winds, brilliant sunshine and cold shadows led me to reflect on the melancholy end of butterflies. We were not observing a functioning reproductive community of butterflies but an asocial agglomeration. Their behaviour was so untypical it could have preceded an extinction and I wondered if the stressed last days of the Large Blue or Chequered Skipper were something like this. It is easy to dismiss these thoughts as an unscientific postscript and yet they are grounded in reality like never before. Scientific enquiry is reaching similar conclusions in a number of fields only to recoil ever anew before the enormities it has laid bare. Retreating behind academic honours and acclaim, the compromised theorists of extremes refuse point blank to raise the social question their work necessarily implies. (C/F 'When Life Nearly Died'- 2003 - by Michael Benton, an account of the greatest mass extinction of all time. In this book the scientific myth that the seas boiled at the end of the Permian era is demolished. Global temperature rose by a mere 8 degrees). A book that came out in 1995, 'Ecology and Conservation of Butterflies', edited by A Pullin in association with Butterfly Conservation, suggests (though not as explicitly as one would have liked) that butterflies are the thin edge of the wedge and their disappearance implies the destruction of a whole swathe of flora and fauna. In the growing wasteland of the 2Ist Century we yearn for those bygone days not so long ago when, paradoxically, it was still possible to make such comforting, discrete, observations. The rise of the scientific study of insects was always accompanied by a hidden anthropomorhism, unspoken social metaphors, vague personifications of a new, different world and a mute desire to change life. As such it was heir to the pathetic fallacy of the romantic movement. However this literary illusion has now become flesh and bone, our flesh and bone, and in the fate of butterflies we can read our own fate. And we make no apologies whatsoever for insisting on this.

 

Wintry Cones