Early June at Newhaven's Castle Hill LNR: continental visitors arrive

Week 10 of the 2016 transect-walking season

 

Butterfly numbers were again lower than expected with only seven species seen during the walk. The weather during the walk was warm and sunny with only light winds, but this was the first day in a while that the weather was not cool, cloudy and windy. Despite the tendency to think that numbers are poor, upon comparison with most other years numbers are about average - and one species, the Green Hairstreak (Callophrys rubi) has already had its best year to date. Eighteen butterfly species have been recorded so far during 2016.

Diamond-back Moth (Plutella xylostella)

Two overseas species spiced up proceedings: the tiny Diamond-back Moth (Plutella xylostella) and the much larger - and much more noticeable - Painted Lady (Vanessa cardui). The latter was seen flying in off the sea in a northerly direction and these are possibly the vanguard of a greater immigration which may occur over the coming days. These are a strong, unmistakeable insect and there is always a hope that the event of 2008 will be repeated, when many, many millions of these butterflies piggybacked across three generations from the African Atlas Mountains into the UK and as far north even as the Arctic Circle, before their offspring made the return journey back south at the end of their season. 

The Diamond-back Moth is in the midst of a similar immigration event, which hit Newhaven on 30th May, but apparently from the east rather than from southerly climes. I have recorded more than ten thousand of these moths in Newhaven during the past week and I saw an additional 400 - well, 398 - during this walk. Many of these were nestled amongst hoary cress and kale along the cliff bottom and their caterpillars will feed on these and other plants of the cabbage family during the summer. There were without doubt many thousands more than the 400 that I counted, but transect-walking rules are not to be broken, so I counted only what flew up before me as I stepped along the route! I have written a separate report about the Diamond-back Moths which provides more information about this tiny species and the current immigration event.

Large Skipper (Ochlodes faunus)

Whilst these and other exotic species add colour to the nature reserve, we are inevitably more interested in the native species which breed each year at Castle Hill. Numbers fluctuate annually, in response to many variable and confounding factors, and not least the weather. These species have evolved to take the knocks and then make hay when conditions are more favourable. This year is proving to be more or less average when compared with some previous years. It was pleasing to see the emergence of the Large Skippers (Ochlodes faunus), albeit in only small numbers, but these feisty little butterflies, which lay their eggs on Cocksfoot grass, should increase in number during the next few weeks. Common Blue (Polyommatus icarus) numbers are increasing by the week and the first brood should peak in the next two or three weeks, as should the day-flying moths the Mother Shipton (Euclidia mi) and Burnet Companion (Euclidia glyphica), which can be found amongst herbaceous areas along the tops of the cliffs.

Earlier in the year I made a chance observation of a Raven raiding the Barrow Head cliff face hollows which are used as nest holes by a small group of Jackdaws and feral pigeons. The Jacks were perturbed and apparently helpless on that occasion as the Raven leered menacingly into each hole, but there was nothing to steal away at the time (egg-laying time was still in the future) and it flew off empty-handed. I witnessed a repeat raid today, probably by the same Raven. The response of the Jacks could not have been more different: whereas they previously appeared to yield helplessly to their preponderant corvine cousin, today they were a tight fighting force. Arranged in formation they swooped in turn, five Jacks deep, to mob their attacker, they swarmed around, jostled, grappled, set upon, overran and comprehensively saw it off. Within the space of thirty seconds the egg thief was utterly routed. It was spectacular stuff! The Jacks had a most valuable resource to lose this time in their chicks, which must now be very close to fledging. I think this is an example of the concept in the study of behavioural ecology known as the incumbency advantage.

Krebs & Davies (1993) eloquently explained at length how an incumbent (or owner of a resource) has an advantage over a challenger because the incumbent knows or understands the resource at stake, whereas the rival does not possess such detailed knowledge. I am not a behavioural ecologist, but I believe the battle over the Jackdaw chicks (the resource) was an example of this. The investment by the several adults and the bonds with the chicks resulted in a much greater challenge to the Raven than when their nest holes were empty. The hypothesis of the incumbent advantage is that the owner will fight more strongly than the less certain challenger and almost always win. Also, they have more to lose than the predator has to gain. The stakes vary and there are plenty of examples in nature when the predator wins the prey, but I was stunned and mightily impressed by the ferocity of the parent birds on this occasion.

The reserve is easing itself into the summer season now. Grasses are shooting skywards and flowering, along with Thrift and the Common Blue's food-plant Bird's-foot Trefoil on and above the cliff faces. There are the usual concerns about humans' conflicting interests and multiple uses of the nature reserve, but I am not going to bore the reader with a personal diatribe until I have taken the time to convey my feelings - my disgust even - in a more positive light.

The full list for Week 10 (24 species)

Butterflies (7 species) Large Skipper (Ochlodes sylvanus)  3
Small White (Pieris rapae)  1
Green-veined White (Pieris napi)  1
Speckled Wood (Pararge aegeria)  2
Small Heath (Coenonympha pamphilus)  1
Painted Lady (Vanessa cardui)  9
Common Blue (Polyommatus icarus) 12
Moths (17 species) Diamond-back Moth (Plutella xylostella)  398
Common Tubic (Alabonia geoffrella)  1
Light Brown Apple Moth (Epiphyas postvittana) 1
Common Marble (Celypha lacunana)  3
Plum Tortrix (Hedya pruniana)  5
Knapweed Bell (Epiblema cirsiana)  1
Regal Piercer (Pammene regiana)  1 – first site record
Grey Gorse Piericer (Cydia ulicetana)  15
Lead-coloured Drill (Dichrorampha plumbana)  1
Garden Grass-veneer (Chrysoteuchia culmella)  1
Meadow Grey (Scoparia pyralella)  1
Little Grey (Dipleurina lacustrata)  2
Lackey (Malacosoma neustria)  1
Yellow Belle (Semiaspilates ochrearia)  1
Angle Shades (Phlogophora meticulosa)  1
Mother Shipton (Callistege mi)  1
Burnet Companion (Euclidia glyphica) 4
Other highlights Grove or Brown-lipped Snail (Cepaea nemoralis)  1 – not previously found at CHLNR
22-spot Ladybird (Psyllobora vigintiduopunctata)  1
Dock Bug (Coreus marginatus)  7
Green Shield-bug (Palomena prasina)  1

Common Tubic (Alabonia geoffrella)

Garden Grass-veneer (Chrysoteuchia culmella)