Published on 01 December 2011 in Letters

Miles (2011) listed the changes in the popu- lation of St Kilda Wrens Troglodytes troglodytes hirtensis, mainly since 1931, on the four main islands of the archipelago and in the village area on Hirta. The variation in the estimates for the main islands is great (for Hirta, the range is 15-157 singing males) and it is important to be clear how difficult (impossible?) it is to compare the various estimates of the different island populations. (In particular, table 1 in Miles (2011) perhaps suggests that the figures are more comparable than they really are – in fact, not all the counts are of singing males, nor are they always ‘whole-island’ counts.) A key point is that the methods used in these surveys were not standardised. The variation is almost cer- tainly more of a reflection of the timing (both time of day and date), the intensity of the survey (both number of surveyors and duration of survey) and perhaps the weather conditions than any real changes in Wren numbers. In view of the difficulty in sur- veying the whole archipelago, the numbers of Wren pairs in the village area on Hirta have been estimated since 1931 and are shown as Miles’s fig. 1. The reasons for the variation in the wider surveys (timing, intensity and weather) also apply to that of the village area and it is difficult to show discrepancies and ranges in a single graph. Moreover, authors have variously considered the Village Glen as a whole or (more usually) as the village area within the boundary wall – some of them conceivably including Wrens that use the boundary wall in their territory but nest outside it, others not. These problems were clearly recognised by Williamson (1958).

Given these problems, it is surprising that the number of Wrens in ‘the village area’
remained more or less between 10 and 12 for eight studies between 1931 and 1952. There was a slight decline in the 1950s and a con- siderable increase around 1990. This relative consistency is perhaps an indication that the village area is easy to study and should con- tinue to be surveyed as often as possible but with clear identification of the area studied. However, we do not know if population changes in the village reflect those of the archipelago as a whole; given the association of St Kilda Wrens with seabird colonies, it probably does not. It would therefore be useful if a convenient section of typical St Kilda Wren habitat was identified and regular surveys were carried out there.

Click here to download the full article.