VOL. LI OCTOBER No. 10 1958 BRITISH BIRDS POPULATION AND BREEDING ENVIRONMENT OF THE ST. KILDA AND FAIR ISLE WRENS By KENNETH WILLIAMSON (Plates 61-6^ (i) INTRODUCTION THE FIRST WRITER to mention the existence of a Wren (Troglo­ dytes troglodytes) on St. Kilda, over a hundred miles west of the Scottish mainland and half that distance from the Outer Hebrides, was Martin (1698); but it was left to Macaulay (1764) to pose the first of innumerable questions which have sprung from the wonder­ ment of naturalists that such a bird should be there at all: "How these little birds, I mean the wrens particularly, could have flown thither, or whether they went accidentally in boats, I leave un­ determined." Later students have found the challenge of the St. Kilda Wren (T. t. hirtensis) irresistible, but the pertinent problem has not been that posed by Macaulay so much as what happened to the Wrens once they were there, and how they accomplished such complete mastery of this unusual environment. Almost the first thing learned about them was that they were different from mainland Wrens, and indeed Seebohm (1884) described them as a species new to science, Troglodytes hirtensis. When Dresser (1886) complained that this was going too far, the St. Kilda Wren was reduced to subspecific rank, a status which fits well the more dynamic approach to taxonomy that is the vogue today. Barrington (1884) seems to have been the first to speculate as to why the St. Kilda Wren should be so different from mainland birds; he suspected that exposure to a more humid maritime climate might have much to do with its greater size and greyer- brown colouring. Plumage descriptions have been given by Clark (1915), Harrisson and Buchan (1934) and Witherby et al. (1938); it has been discovered that the St. Kilda bird has a unique song (which was sound-recorded in 1956 and 1957), and its nesting- behaviour has been studied by Harrisson and Buchan (1934, 1936) and Bagenal (1958). Study of the differentiation among this 369 370 BRITISH BIRDS [VOL. LI and other insular communities of Wrens in the north-east Atlantic region has received a great stimulus in recent years through the researches of Armstrong (1952, 1953, etc.). Knowledge of the situation in this one species has thrown new light on problems of evolution in general, and it is perhaps for this reason more than any other that continued studies of island communities of the Wren are important. Some years ago I drew attention to the fact that the Wrens of Fair Isle, between the Orkney and Shetland Islands and 25 miles from each, were distinct from both the typical race and the Shet­ land subspecies Troglodytes t. eetlandicus, and I gave the Fair Isle Wren the name of Troglodytes t. fridariensis (Williamson, 1951). Its distinctness from the typical race is less pronounced than in hirtensis and zetlandicus, but the differences in plumage are constant and incline more to the former than the latter. There can be little reasonable doubt that the Fair Isle community is well insulated from the rest, so thai, like the St. Kilda Wren, it is a suitable subject for a population study. As I was to be at both St. Kilda and Fair Isle for periods during the spring of 1957 the opportunity presented itself for gaining a better understanding of the numbers and breeding environment of these two isolated insular populations. (2) THE ST. KILDA WREN Previous Estimates of the Population. Barrington (1884) came across only 6 Wrens in a 3-weeks' stay in 1883, which is surprisingly few since he was a climber and spent much time on and near the cliffs. Dixon reported them as common in 1884, but in the following year the Factor, J. T. Mackenzie, told him that the birds were not nearly so numerous (Dixon, 1885). Mr. Fiddies informed Elliott (1895) that "it was once far commoner than it is at present, especially about the village . About 15 pairs would fully represent their numbers on the island now". This must surely have been an under­ estimate since Elliott himself found or was shown 3 nests. Never­ theless, we must bear in mind the possibility that considerable periodic fluctuations take place; and in any event some decrease in numbers may well have taken place at this time, for at the turn of the century the Wrens (and in particular the accessible ones in the Village) were persecuted for their skins and eggs. (A letter from a St. Kildan in the Norwich Castle Museum, dated' 1904, asks for £1 is. each for adult skins and 12s. 6d. each for juveniles.) However, the fears of Hudson (1894) and others that the race was ever near extinction were quite unfounded, as Fisher (1948b) and Armstrong (1953) have shown. Serious attempts to census the Wrens began with the 1931 expedition, which recorded 68 pairs for the archipelago—45 on Hirta, n on Dun, 9 on Soay and 3 on Boreray (Harrisson and Buchan, 1934). Cockburn, who had been at St. Kilda for several VOL. LI] ST. KILDA AND FAIR ISLE WRENS 371 months in 1927 and 1928, and whose study of the geology had given him an intimate knowledge of all the islands, told this expedition that he estimated the total Wren population at under a hundred pairs. Eight years later Nicholson and Fisher (1940) attempted to repeat the survey but could not get on to Soay; they counted 48 pairs, with 31 on Hirta, 12 on Dun and at least 5 on Boreray. In a painstaking survey in July 1948 Ferguson-Lees (in litt.) recorded 48 pairs on Hirta and 14 or 15 on Dun, whilst in early August of 1952 about 20 were found on Dun and many on Hirta (Bagenal, 1953). Census work in other years concerned the Village only, and is discussed under the appropriate sub­ heading below. The 1957 Census of Hirta. The above surveys, except for some conducted in the Village area, are open to the serious criticism that they were based on birds heard singing in the daytime and late in the season, when FIG. 1—MAP OF HIRTA, ST. KILDA, SHOWING THE MAIN CONCENTRATIONS OF WRENS (Troglodytes troglodytes hirtensis) IN 1957 Wren positions are indicated by the little circles (not to be confused with offshore stacks and islets). Note the concentrations on Cam Mor and Mullach Bi, on Ard Uachdarachd and Conachair, on Oiseval and in the Village region (see Page 373)- The inset shows the position of the smaller islands of the St. Kilda group in relation to Hirta. 372 BRITISH BIRDS [VOL. LI Wrens are primarily engaged in their domestic affairs and song is desultory. To get a valid idea of numbers and a true picture of distribution a dawn-chorus count before the onset of nesting, in middle or late May, is essential; for only then can one be reason­ ably sure that the potential breeding males, consolidating their territories, will challenge each other with vigorous bursts of song. As Hirta has about 8 miles of coastline and most of the Wrens are on the cliffs, it was clear that such a survey would take at least a week, examining the coast piece by piece on successive days, fair weather permitting, in the 2 hours or so after sunrise. My counts began on 21st May and continued until the 31st, the last being a re-count of Oiseval to get some idea of the margin of error to be expected along this and similarly difficult reaches; they were made between approximately 04.00 and 05.30 hours G.M.T., the only exception being the 29th when there was much ground to cover and I began at 03.00 hours. It is perhaps unnecessary to give the several routes in detail: these, with the resulting counts, have been plotted on the one- inch Ordnance Survey map and deposited in the St. Kilda Reserve Record at the Nature Conservancy, 12 Hope Terrace, Edinburgh, for the use of anyone who might wish to repeat the census in future years (Fig. 1). Counting from the cliff-tops (many of which are well over a thousand feet above the sea) is un­ satisfactory, for in places there are rock outcrops and talus slopes on grassy terraces between the rocky crest and the sea-cliffs below, so that one finds a complicated distribution with Wrens singing among the crags near the top, Wrens challenging from the talus, and Wrens whispering far below on the final low cliff-fall to the sea. This is especially so of Oiseval, the vast rocky slopes of Mullach Bi on the west coast, and the granite buttress of Ard Uachdarachd which flanks the 1,350-foot precipice of Conachair. Good results can be obtained only by contouring the cliffs wherever the angle of slope and the existence of sheep-paths allows, and in order to cover the west coast in this way at dawn it is necessary to sleep out in one of the cleits* on the boulder-field of Cam Mor. The counts are given in Table I, taking the main cliffs and coastal slopes in sections west-about from Village Bay. From this Table it is seen that over 85% of the population of Hirta inhabits the cliffs and coastal slopes, which compares with 82% given by Harrisson and Buchan (1934). Of the inland pairs 11% are confined to Village Glen and just over 3% to Gleann Mor, where none was recorded in 1931. Density and Habitat. I found that the Wrens are by no means uniformly distributed around the coast.