Into the Mountains (part 2): a scarce mountain moth

 

Black Mountain Moth - Beinn a'Chlaidheimh 22.6.24

(Post updated 10 July 2024)

As mentioned in the last blog post (link here), I like to get out into the mountains. Seeing the wildlife of the uplands is an added bonus. In Scotland, that means frequent sightings of Red Deer, Ptarmigan and Mountain Hares, as well as rarer glimpses of Dotterel and Snow Buntings. I have also been lucky to have a couple of encounters with a moth that rarely descends from the higher peaks. To see one, you need to get your boots on.

I'll start by correcting a possible misunderstanding. You will not find a Black Mountain Moth Glacies coracina (previously Psodos coracina) in the Black Mountains of the Welsh-English borderlands. It is, instead, a black moth that lives on mountains. In fact, its distribution is very limited - within the UK, Black Mountain Moths are restricted to the Scottish Highlands (see NBN distribution map here), usually over 600 metres in altitude. The distribution map shows clusters in two main areas of the Highlands - the Cairngorms (in the east) and the hills around Loch Cluanie and Loch Shiel (in the west). However, I have walked both of these areas on many occasions without seeing one. Both of my Black Mountain Moth encounters were further north, in the Fisherfield wilderness to the south east of Ullapool. I found my first one on the rocky shoulder in the foreground of the picture below; this is Mullach Coire Mhic Fhearchair, which Munro baggers will know as one of the 'Fisherfield Five'. It is awkward to get to: our expedition involved an overnight camp in the glen below. The mountains in the background are the more familiar Torridon peaks, with Slioch the most prominent. 

View north from Mullach Coire Mhic Fhearchair - Black Mountain Moth habitat

The second encounter, last month (June 2024), was on another Fisherfield mountain (Beinn a'Chlaidheimh), when my hillwalking colleague Richard spotted the individual pictured at the start of this blog as we were descedning from the summit. It is highly likely that the relative absence of Black Mountain Moth records in the Fisherfield area is a result of the relative absence of moth recorders on these remote mountains! Needless to say, I have submitted both of my records to the relevant County Moth Recorder, so hopefully a couple more dots will be added to the map in due course.

The first moth that I found (below), one of two that I saw on that walk, was flying low above the ground. This must be a behavioural adaptation to the windy conditions of their preferred habitat. The females tend to stay on the ground, although they are not wingless like some other species. I have only seen males; their flight action makes them more conspicuous. Not a lot else is moving around at that altitude.

My first Black Mountain Moth - Mullach Choir Mhic Fhearchair 19.6.21

Black Mountain Moths have a number of other adaptations to life in the uplands. Their black colouration comes from the pigment melanin which helps to absorb heat more efficiently as well as protect from UV radiation - 'thermal melanism' (Majerus, 2002; Lees & Zilli, 2019). Their furry bodies provide extra insulation. The black colour is also likely to provide some camouflage for these day fliers when seen against the generally dark backgrounds of the peaty and rocky mountain habitats.

A further possible adaptation is discussed in E.B.Ford's classic New Naturalist Moths (Ford, 1955), which states that Black Mountain Moths are more common in odd years of the calendar, with the Northern Dart - an upland moth that I have yet to track down - being more easily found in the even years. He comments that while it is understandable that a high mountain moth might take longer to develop and emerge, the establishment of a two year cycle remains 'mysterious'. Waring & Townsend (2017) say that the two year cycle is 'almost certain' because 'on some sites the adults appear to be more numerous every other year, generally in odd-numbered years'. However, a quick literature search has found no recent work that sheds light on this matter. Indeed, one rather obscure paper (Kaaber, 1996) states that Black Mountain Moth shows no alternate year rhythm in northern Scandinavia. So I wonder how robust this observation really is: for what its worth, my own small sample of records were in both odd and even years. It would be interesting to examine the data in more detail.

Beinn a'Chlaidheimh North Ridge - site of the June 2024 Black Mountain Moth encounter

The global distribution of  Black Mountain Moth emphasises its preference for upland habitats (link to GBIF distribution map here): it is found primarily in Scandinavia, the Alps and the Pyrenees, so its UK population is a bit of an outlier. There is some evidence that, like birds such as the Ptarmigan, it is found at lower altitudes at higher latitudes.

The restricted distribution of Black Mountain Moths - particularly in Scotland - suggests that the species may become a conservation concern in the light of a warming climate. If so, this hasn't been recognised by its inclusion as a UK Biodiversity Action Plan species - although that list is now looking rather dated. (The rarer Netted Mountain Moth is identified as a BAP priority species.) But, short of general climate change policies, its difficult to know what specific actions could be undertaken in respect of the Black Mountain Moth. Its habitats are generally well away from the pressures of development or intensive agriculture. Passing hillwakers generate limited disturbance. The main larval foodplant (Crowberry Empetrum nigrum) has a wider range within the Scottish Highlands than the moth, implying that foodplant availability is not the limiting factor for the moth's distribution.

At the moment, no population trends are available for Black Mountain Moth; the excellent Atlas of Britain and Ireland's Larger Moths notes that there has been a dramatic increase in records over the ten years to 2019, partly as a result of hillwalkers with mobile phones. This suggests greater recording effort rather than an expanding population, but at least numbers of the moth appear to be presently secure. Nevertheless, this species is one to watch carefully in the future.

Acknowledgements

Thanks to Richard Leslie for locating the second Black Mountain Moth on Beinn a'Chlaidheimh and to Phil Sterling for a useful discussion about the species.

References

Ford, E.B. (1955) Moths. London: Collins New Naturalist.

Lees, D.C. & Zilli, A. (2019) Moths. London: Natural History Museum.

Kaaber, S. (1996) 'Notes on Xestia alpicola ssp. atlantica nov.( Lep., Noctuidae) on the Faroe Islands.' Fróðskaparrit-Faroese Scientific Journal, pp.107-113.

Majerus, M. (2002) Moths. London: Collins New Naturalist.

Randle, Z., Evans-Hill, L. et al. (2019) Atlas of Britain and Ireland's Larger Moths. Newbury: Pisces Publications

Waring, P., Townsend, M & Lewington R. (2017) Field Guide to the Moths of Britain and Ireland (3rd ed.) London: Bloomsbury.