A further sunny forecast for much of yesterday afforded an opportunity denied so far this year to survey my “home reserve” Aston Rowant NNR (N) for spring butterflies. The top target was the iconic Adonis Blue that has established a reliable colony on the hillside above the M40 since I first observed them there in September 2012. But so far in 2015 I had not recorded other key species such as Brown Argus and Small Heath in Oxon, Bucks and Berks (BC UTB area) and that also needed to be addressed.
The downside of this site is that cloud often stacks up on the Chilterns escarpment, so butterflying can involve a little too much waiting for the sun to break through. This morning was no exception and hence in the first couple of hours I found just a few each of Common Blue and Brown Argus and a lone Dingy Skipper. While waiting on a slowly approaching patch of blue sky I walked a circular route around areas of the reserve that I don’t usually get to, being reminded of the sheer quantity of superb butterfly habitat that exists here. I timed this excursion well, arriving back at the area where I expect to see Adonis Blue just after the sun had broken through, but none of that species were to be seen.
Just after midday blue skies came to prevail and though I was two-thirds of the way back to the car park I just had to go round again. With the hillside now bathed in sunshine Common Blue and Brown Argus (that is another blue) were suddenly flying in great numbers, and some Green Hairstreak were emerging at the brambly foot of the hillside. I also found a first Small Copper of 2015 and first in-area Small Heath as I retraced my steps to the Adonis hot spot, but a second scan there was no more productive than the first.
Then on going back through the gate at the end of the sunken way trail, there were three male Adonis Blue on the wing. In the picture (above) I usually expect to see them on the far side of the fence that runs down the hillside. But yesterday they were on the near side where though the grass was longer there was plenty of the food plant Horseshoe Vetch. Two of these butterflies allowed a very close approach, when they weren’t being harassed by aggressive little Brown Argus, and hence mission was accomplished with good pictures.
The double-brooded (late May and August) Adonis Blue is localised throughout its range on southern English chalklands. The vivid colouring of the males usually makes them stand out at some distance though this can range from turquoise to violet, confusing separation from the deeper violet and far more numerous Common Blue. The fine black lines crossing the outer white fringes and just entering the body of the wings, that the above picture clearly shows, are a diagnostic feature. The year’s first Adonis Blue were also reported at two more key BC UTB sites, Lardon Chase (Berks) and Yoesden Bank (Bucks) on 27th May.
I know of several sites in south Oxon where the diminutive but fascinating Small Blue may be found. My favourite hot spot is just south of Lowbury Hill on the border with Berkshire, the highest point on the downs above Blewbury. But this involves a 5-mile walk out and back that just invites sod’s law to roll in the grey stuff upon arrival. Today was not one to risk it so instead I visited the easiest accessible site Hagbourne Railway Embankment (SU622894). This former branch line, now a designated cycling route and permissive footpath, is a gem of a butterfly site in high season being very wild flower rich. And it has two specialities: Essex Skipper and Small Blue.
Though widely distributed the latter has a need for sheltered locations, old quarries and steep embankments being favoured habitats. Blustery conditions this afternoon meant they were unlikely to be flying on the western side of this particular embankment where I usually find them. But about halfway along the route is a feature (pictured above) where tarmac access paths split the eastern slopes. Hence the more sheltered habitat here was easy to scan and indeed I found two of Britain’s smallest butterfly. These were difficult to capture in the long grass but with persistence I gained an acceptable record shot (below).