Short paper The appearance and status of the St Kilda Wren Abstract This short paper summarises all known data on the population size and trends of the St Kilda Wren Troglodytes troglodytes hirtensis, now included as a taxon monitored by the Rare Breeding Birds Panel. The appearance of this distinctive subspecies of Wren is discussed briefly and illustrated. The St Kilda Wren Troglodytes troglodytes and more distinctly barred than the other UK hirtensis occurs only on the archipelago after subspecies, with the head and upperparts dis- which it is named, which lies approximately tinctly greyish, the rear flanks heavily barred 70 km west of Harris, in the Outer Hebrides, blackish-brown, and throat and breast rela- and comprises four main islands: Hirta, Dùn, tively unmarked pale grey (e.g. Seebohm Soay and Boreray. The St Kilda Wren has the 1884, Clarke 1915, Harrison & Buchan 1934, most distinctive appearance of all the sub- Brewer 2001, Collinson 2002, McGowan et al. species of Wren in Britain, being on average 2004). In 1931, Harrison observed that St bigger-bodied, longer-billed and with paler Kilda Wrens show a marked colour dimor- plumage than the Fair Isle Wren T. t. phism, with distinct light and dark plumage fridariensis, Shetland Wren T. t. zetlandicus, variants (Harrison & Buchan 1934; Love Hebridean Wren T. t. hebridensis, and UK 2009). mainland Wrens, T. t. indigenus and T. t. During the course of many months living troglodytes. Love (2009) presented a useful on St Kilda during 2007–10, I saw the island comparison of the mass and body size of St Wrens frequently on Hirta, particularly at Kilda and UK mainland Wrens (the latter in Carn Mor and in the village. I was initially parentheses): body mass 12.5–14.5 g surprised by their appearance and plumage, (8.9–11.0); wing length 48–52 mm, mean 50.2 being far more distinctive than I was (45–52, mean 47.5); tarsus length 18–21 mm expecting. In the field the birds looked start - (16.6–19.1); bill length, to skull 13.5–16.0 mm lingly large, much bigger – and longer-bodied (13.3–14.8). The plumage of St Kilda Wrens – than the other UK subspecies, as well as has been described by many as paler, greyer very long-billed. The general impression Will Miles 180. Adult St Kilda Wren Troglogytes troglodytes hirtensis, Hirta, St Kilda, Outer Hebrides, July 2009. The cold, pale grey tones of the head, underparts and mantle barring of this subspecies closely match the muted colours of its favoured habitats of rocky sea cliffs, boulder scree, and derelict stone walls. Note the dark, blackish barring on the rear flanks and orange-tinted tail and wings. © British Birds 104 • June 2011 • 325–328 325 Short paper seemed unfamiliar, more creeper-like than and secondaries were contrastingly orange- wren. Most distinctive in adults was just how tinted (plates 180–182), forming a pale pale, cold and stony were the grey tones of orange panel in the closed wing. I did not the head, underparts and mantle. In addition, encounter any distinct dark plumage variants the tertials and outer webs of the primaries among adults and only slight individual vari- ation in the barring of the upper- parts was noticeable. However, juveniles were considerably darker than adults (plate 183). Based on the plumage of four juveniles ringed on Hirta in 2008, plus other, unringed individuals, young St Kilda Wrens can be identified in the field by their dark plumage until at least November of their first year. The yellow bill ‘gape’ of young juveniles was present from fledging (in late June/early July) until late August. In addition, one adult bird was trapped in June 2008 (plates 181 & 182); all meas- urements taken (body mass 14.9 g; wing length 56 mm; tarsus length Sarah Sarah Money 22.3 mm; bill length, to skull 17 mm) exceeded the upper limits given by Love (2009). Complete survey coverage of the breeding birds of St Kilda is particularly challenging, owing to difficult weather conditions and the fact that island landings are often impossible. The number of whole-island surveys of singing male St Kilda Wrens reflects this (table 1). Not only are surveys Will Miles generally few, but only in 1931 and 1957 have estimates of the number of singing males been made for all four of the main islands in the same year. The 1957 estimate of approx. 230 pairs has often been referred to as a likely minimum population size for the archipelago. However, it is impor- tant to note that Boreray, Soay and Dùn were not actually visited and surveyed for Wrens in 1957; the population estimates for those islands, and Stac an Armin, were derived from a study carried out Will Miles 181–183. St Kilda Wrens Troglodytes troglodytes hirtensis, entirely on Hirta (Williamson Hirta, St Kilda, Outer Hebrides; adult (top and centre, 14th 1958). In comparison with 1957 June 2008) and newly fledged juvenile (bottom, 2nd July 2008). estimates, the numbers of Wrens 326 British Birds 104 • June 2011 • 325–328 Short paper actually recorded on Boreray, Soay and Dùn in Table 1. Numbers of singing male St Kilda Wrens Troglodytes troglodytes hirtensis heard in spring (May–July); St Kilda archipelago other years are invariably total estimates and whole-island counts for Hirta, Dùn, Soay and lower, and the 1957 figure Boreray, 1931–2010. These represent, so far as is known, all of 233 pairs for the archi- published estimates of the population. pelago should perhaps be treated with caution. Year No. singing Source males Another consideration is that many of the islands’ St Kilda archipelago steepest cliff faces have 1931 68 Harrison & Buchan 1934 enormous areas of appar- 1957 c. 233 Williamson 1958 ently suitable breeding Hirta habitat, yet these are often 1895 c. 15 Elliot 1895 impossible to get to and 1931 45 Harrison & Buchan 1934 census accurately for 1939 31 Nicholson & Fisher 1940 birds (pers. obs.). A 1948 c. 48 James Ferguson-Lees (in Murray 2002) modern survey of the 1957 c. 116 Williamson 1958 entire archipelago would 1962 92 Waters 1964, Love 2009 not only provide an 1990 145–157 Murray 2002 1993 113–117 Vaughan & Love 1994 extremely useful indicator of the global population Dùn of this distinctive sub- 1931 11 Harrison & Buchan 1934 species but be a very con- 1939 12 Nicholson & Fisher 1940 siderable achievement. 1948 14 or 15 James Ferguson-Lees (in Murray 2002) On a smaller scale, Village 1957 c. 25 Williamson 1958 1977 20 Harris & Murray 1978 Glen on Hirta has been used relatively frequently Soay as a sample site for sur- 1931 9 Harrison & Buchan 1934 veying breeding Wrens. 1957 c. 45 Williamson 1958 Since 1931, the number of 1989 8 Moore & Rothe 1989, Murray 2002 singing males within the 1993 13 Vaughan & Love 1994 1999 12 Murray 2002 village boundary wall has mostly remained low, Boreray fluctuating between five 1931 3 Harrison & Buchan 1934 and 12 individuals, 1939 5+ Nicholson & Fisher 1940 although it peaked at over 1957 c. 45 Williamson 1958 20 in the 1990s (fig. 1). 1980 9 Duncan et al. 1982 1990 12 Murray 2002 Armstrong (1953) and 1993 10 Vaughan & Love 1994 Williamson (1958) noted that Wren nesting terri - Stac an Armin tories were especially 1957 c. 2 Williamson 1958 abundant on the rocky slopes at Carn Mor and on Dùn in compar- pelagicus, Manx Shearwaters Puffinus puffinus ison with the village on Hirta, and suggested and Fulmars Fulmarus glacialis (Mitchell et al. that this was due to the presence of large 2004; Newson et al. 2008). The occurrence of colonies of Puffins Fratercula arctica at these Wrens nesting alongside one species of auk two sites, and an abundance of food (inverte- and four species of petrel, and perhaps greatly brates) associated with Puffin guano, chick benefiting from this situation, is unique corpses, and dropped and rotting fish. We within the UK, and further research would be now know that, as well as Puffins, Dùn and useful on the feeding and breeding ecology of Carn Mor hold very large colonies of Leach’s St Kilda Wrens, with special relation to Storm-petrels Oceanodroma leucorhoa, and seabird colonies. Indeed, overall, this distinc- also European Storm-petrels Hydrobates tive British endemic remains relatively little- British Birds 104 • June 2011 • 325–328 327 Short paper The birds of Boreray, 30 St Kilda. Seabird Report 6: 18–25. 25 Elliot, J. S. 1895. Observations on the fauna of St Kilda. 20 The Zoologist 19: 281–286. 15 Fisher, J. 1948. St Kilda: a natural experiment. 10 New Naturalist Journal 1948: 91–108. Harris, M. P., & Murray, 5 S. 1978. Birds of St Kilda. ITE, Cambridge. 0 Harrison, T. H., & 1931 1941 1951 1961 1971 1981 1991 2001 Buchan, J. N. S. 1934. A field study of the Fig. 1. Numbers of singing male St Kilda Wrens Troglodytes troglodytes St Kilda Wren hirtensis heard in spring (May–July) within the village on Hirta, St Kilda, (Troglodytes 1931–2010. Data from the following sources: Harrison & Buchan (1934), troglodytes hirtensis), Harrison & Lack (1934), Nicholson & Fisher (1940), Fisher (1948), Atkinson with especial reference to its (1949), Armstrong (1953), Boyd et al. (1956), Williamson (1958), Waters numbers, territory (1964), Vaughan & Love (1994), Murray (2002), Love (2009), plus various and food habits. annual reports of the St Kilda Ranger (unpublished reports to Scottish J. Anim. Ecol. Natural Heritage and the National Trust for Scotland, 1970–2010). 3: 133–145. — & Lack, D. 1934. known, with many critical questions still to be The breeding birds of St Kilda.