INDIVIDUALS OR SIGHTINGS?

My last two visits to Steyning Rifle Range on 20th and 22nd September produced 6 and 7 female Brown Hairstreak respectively, bringing my 2012 total for the site to 52. To give some idea of how prolific the species is here, this is a lower count than in previous years; 2012 being a modest or even poor season in most areas.

One of the questions I'm regularly asked is "how do you know whether you are counting the same butterflies more than once?" Clearly it is useful to know whether we are recording sightings or different individuals, bearing in mind that the data will be useful in assessing the fortunes of the Brown Hairstreak from year to year, and the effectiveness of any management being conducted for the species.

Recognising different individuals is much more difficult early in the season, particularly when females are waiting for their eggs to ripen and are yet to begin their regular descent deep into the Prunus for the purpose of oviposition. Assuming that good images can be captured for each individual, a close examination will usually reveal subtle differences in pattern, particularly in the shape and extent of the orange wing flashes and the underside ‘streaking’. Also, unless the butterfly has emerged very recently, there will often be the odd telltale hairline scratch to the otherwise perfect topside.

Once the females start egg laying damage starts to occur very rapidly, which is unsurprising given their constant manoeuvring amongst the thorn. Aside from the scratches and loss of scales which are rapidly suffered, the Brown Hairstreak picks up a highly characteristic pattern of damage to the wing margins, unlike that seen in any other species. Semicircular notches are soon picked up as the wing margin folds and breaks against thorns and stems. These often develop further into deeper tears as the butterfly reverses up against Prunus stems while testing for suitable oviposition sites. Typical thorn damage can be seen in some of Andy Barker's images of 17th September. Later in the season this type of wear & tear allows easy differentiation of many individuals.

 

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