Foreign Travel – Wiltshire! Sidbury Hill Fort near Tidworth.

Sidbury Hill

6th May, 2020.

David Beeson and John Solomon

David

Flora

Sidbury Hill Fort is across the Hampshire county boundary and is located in Wiltshire, just west of Tidworth. It is mainly Iron Age in construction but earlier settlement remains have been found. If you visit, you’ll encounter two substantial ditches and the fort is located in an elevated position. Much of the site is treed, but the slopes are mostly clear of woody vegetation.

The fort is located on MOD land but access is safe … but avoid the tanks!

Parking is possible on a closed section of the A338 just north of the military cemetery. GR 233 509. From there a footpath leads past Pennings Plantation to the hill fort.

Once on the ramparts there is an extensive view northwards over training lands mostly comprising grassland.

Ash dieback was well advanced when the site was visited. Almost all the black-tipped ash trees were dead or their spring growth showed on only some woody branches.

Other trees included sycamore, ancient spindle plants, wayfaring (Vibernum lanata) in flower and may (hawthorn) blossom coats the spiky shrubs. Blackthorn is common.

The soil, of course, is built from the underlying chalk. Not that it is lying down deep here. The soil classification is: rendzina. This is characterised by a thin surface layer, no sub-soil, and then pure chalk. The soil depth can vary but will naturally be only a few centimetres in depth. Obviously, it will be alkaline – pH 8 or higher, free draining and warming quickly in the sun. Plants that dislike these conditions will be absent or only showing in patches where there has been some acidification. (There are a few acid-loving gorse bushes.)

Plants growing in calcium-rich conditions are called calcicoles. Calcifuges dislike calcium soils. Wild clematis (Old man’s beard) is a calcicole. Hampshire’s heathers are calcifuges.

Despite the chalky soil and the open nature of the hill slopes the vegetation was less diverse than I had anticipated. With April and May being warm, sunny and with a touch of rain the plants have had plenty of encouragement to show themselves.

Cuckoo flower

There were still primroses and bluebells in flower. Cuckoo flowers were attracting the orange tip butterflies, elsewhere I discovered one substantial patch of sorrel and yellow rockroses along the track. I spied no thyme and members of the pea family (vetches) were thin-on-the-ground.  

Day-flying moth

The lower parts of the site are dominated by long and tangled grasses that have excluded the more timid plants. These grasses keep company with well-established shrubs – especially hawthorn (may). Here there must be a superb vole, shrew and mouse population – except they seldom show themselves.

Early purple orchids

Many visitors keep an eye out for wild orchids. Today there were a few dozen early purple orchids in flower and a single spotted orchid was in leaf, yet one might have expected greater numbers and diversity. Perhaps I just missed the better locations. I’ll try again another day.

Garden rockroses

Sidbury Rings, 6 May 2020

John

Invertebrates

We park the cars down a small cul-de-sac, just off the A338, that acts as the drive-way to a couple of houses and park well out of the way. It’s a beautiful late-Spring day with a cloudless sky and the temperature settling nicely into the very low twenties. We can see the Hill Fort rising up in front of us, perhaps a mile away, and we set off along a very deeply rutted track. There are Hawthorn flowers out, here, although I see no butterflies being drawn to them, but it isn’t long before I start to see the occasional Brimstone and Orange Tip.

I haven’t been to Sidbury Rings before so all I have to go on is what I can see from the map. It lies on the Eastern edge of Salisbury, which automatically makes it very interesting as the Plain is a vast area of land left to do as it pleases, excepting for the occasional visit from a bunch of guys in khaki trying to blow it all up! This makes it one of the most valuable nature reserves in the UK. From the map I see that a lot of it is covered with woodland, but there is also a large amount of grassland, which will be calciferous and home to some the of area’s more iconic Spring species of butterfly. It may be relatively early in May but I can’t help but be cautiously excited.

After a while we start to climb up towards the Hill Fort proper. So far we haven’t seen anything very special, a Peacock joining the steady trickle of Brimstones and Orange Tips, although I did get a pretty good sighting of a single Dingy Skipper. A bit of a surprise as they tend to prefer rough and scrubby grassland rather than the meadow-like lush stuff growing alongside the track. Up through the shade of some trees and grassland opens up around us. Having, as I say, checked the map I had high hopes here, but it is the wrong kind of grassland for the species I am hoping to see. I need the rough stuff, with scrapes that encourage the growth of Bird’s Foot Trefoil, Rockrose, ideally the Primulas, such as Primroses and Cowslips. Here the grasses are tall, thick and lush. Possibly good for species such as Dark Green and Marsh Fritillary, which will feed on Violets tucked away underneath the cover the grass provides, along with species whose caterpillars feed on various grasses, such as Meadow Browns and Marbled Whites. However, they won’t be along for several weeks, so I hope the Fort ramparts will provide something more encouraging.

We reach a long section of track with a light uphill gradient and a large spread of grass-covered hillside to the left. I step off the track a few yards and walk up parallel to it to see if I can find something or even disturb it. I am rewarded by a small flutter that settles right by me. A Small Heath:

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This photo is not of the insect I saw, it landed down in the grass and I couldn’t get to it, but comes from my own archive and was shot on Stockbridge Down last year. It is a fairly common butterfly, especially on grassland sites such as hill forts, but this one is early with mid-May being a more usual time for the first ones to emerge. Unsurprisingly the caterpillar food plant is various grasses.

Climbing more steeply the track becomes shaded for thirty yards or so and emerging into the sunshine I catch a glimpse of something that really is more interesting. Almost impossible to follow against the background of leaves, grasses, flowers and twigs as the beating of its wings flashes from the russet of the topside to the emerald green of the underside. It lands and I move slowly in. It doesn’t allow me the perfect shot I want, growing bored with my company and flying away, but it does let me have a reasonable snap:

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A Green Hairstreak. However often I see them I am always stunned by the pureness of that green underside to the wings which they are always so proud to show. Fresh ones can look almost metallic if the light catches just so. Butterflies need to raise their body temperature and so their flight muscles in order to take to the air. Being insects and cold-blooded they rely on the sun to warm them up. When they first emerge from the chrysalis that hides away their magical metamorphosis their wings are mere crumpled bags on their backs. They pump them up by forcing bodily fluid, their equivalent of blood, through the veins of the wings, then draw the fluid back into their bodies and allow the sun to dry the wings into their final glory. Those empty veins can now act like the reverse of a radiator. By sitting with the wings open the insect can harvest the warmth of the sun, and that is how they warm those flight muscles up, and that is why you see them basking. The Green Hairstreak, however, far from opening those wings up to collect the warmth actually keeps them closed and, instead, turns sideways on to the sun  and collects the heat that way. It means you always see them as tiny, bright, emerald triangles, but never see the upper russet brown surface. The food plant is Rockrose, commonly found covering mounds and old anthills on calciferous grassland.

Following the track up and round to the right and the earthworks of the hill fort appear before us. A quick scramble and we are on the outside of the ramparts. This looks a lot more promising. The grass here is far shorter and sparer with a much richer selection of other plants and much more variability in the terrain. Looking to the right the sun is beaming straight onto the inside incline of the ditch and I drop down to the bottom and start walking slowly round. It isn’t long before I start finding Dingy Skippers:

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The food plant is Bird’s Foot Trefoil and I am not finding many examples of this small yellow-flowered plant. Officially, there is no alternative food plant  but I visit a site on private property on a regular basis and, while it is a very small site, I regularly make a count of around a hundred even though I have only found two very small patches of it there. Nature has a habit of throwing up conundrums! Perhaps an alternative is a discovery waiting to be made. Along this small length of incline, perhaps seventy five yards, I count 5 specimens. That is quite low for such a site as this, but, as I say, there seems virtually no Bird’s Foot Trefoil.

I climb up the inside bank and am faced with more meadow-type grassland and a lot of hawthorn. After ten minutes David calls out to me says he had found another Green Hairstreak, so I hurry over. Of course, it has gone, but it isn’t long before I find another one, then another. They are definitely here, but, again, I am finding no Rockrose food plant. I soon see another, then another. I could have seen four, but one of them could be a double count, so I shall record it as three. I notice that while there is scant Rockrose there is a noticeable amount of Gorse and this is also a recognised food plant for this species. Perhaps they are using that.

Then, a flash of Orange! I recognise it quickly from the size, medium for this country, the time of year and the strong flight pattern. A Wall Brown:

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This used to be a common butterfly but much of the wild grassland it favours has now been built on or ploughed up and they are becoming very localised. The caterpillar is not a fussy eater, happily munching on a variety of wild grasses, but, as with any wildlife, if the environment they need is destroyed then the species will no longer be able live there. This one is a female, although the markings are very heavy, the male having a further dark arc on the front wings.

This stretch of rampart, and we are three quarters of the way round the Fort, rapidly becomes the most productive. There are plenty of Orange Tips and Brimstones but also I am finding the occasional Wall, along with regular sightings of Dingy Skippers. Then David, who has been searching the site for his speciality of flora, finds a rather handsome specimen of Early Purple Orchid. We both take turns photographing it. While we are doing so I wonder whose photo he will use to illustrate this blog entry. I think I know the answer to that:

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A little further on and the flickering flight of another small butterfly. Immediately I spot that this isn’t a Dingy Skipper and, when it lands, my identification is confirmed. A Grizzled Skipper:

This is a female.The food plant is wild strawberry, but, again, I haven’t noticed much here at all. Perhaps that explains why I have only found one.

Now we are back where we entered the ring and it is time for the mile long trek back to the cars. It’s been a lovely afternoon but I can’t help feeling a little disappointed. I found the species I expected to find, the Wall butterflies were the high for me, but I didn’t really find the numbers I anticipated. It could be that it is still a shade early, those butterflies we have seen are the vanguard, but I was also surprised at the paucity of the food plants they require on a site that has been untouched for so long. I did however, see quite a lot of Blackthorn around, the food plant for the Brown Hairstreak, so perhaps a visit at the beginning of August could be in order. Salisbury Plain is an area renowned for them.

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