Birds of the Shiant Islands, Outer Hebrides
by M. de L. Brooke
This report of two Cambridge University expeditions to the Shiant Islands draws together their own and previous records concerning the bird-life of the islands, and presents the results of attempts to count the sea-bird populations.
RECENT INFORMATION on the birds of the Shiants (Outer Hebrides) is confined to references in papers dealing with individual species on a national scale. This paper attempts to bring these together, and add the observations of two Cambridge University parties which visited the islands from 20 June to 8 July 1970 and from 19 June to 3 July 1971. In 1970 the primary aim was to count the seabirds for 'Operation Seafarer', since the Shiants were missed in the main year of the survey, 1969. In 1971, further counts and observations were made on the auks. In the case of the Puffin Fratercula arctica this work has been published separately (Brooke 1972).
The Shiant Islands, which have an area of about 230 hectares (570 acres), lie in the Minch approximately 8 km. (5 miles) southeast of Lewis and 21 km. (13 miles) north of Skye. The group consists of three main islands and a string of islets, the Galtachean, to the west, as shown in Figure 1. Garbh Eilean and Eilean an Tighe are joined by a narrow shingle isthmus, and a small uninhabited cottage is situated on Eilean an Tighe. To the east is Eilean Mhuire.
Garbh Eilean and Eilean an Tighe are both largely composed of columnar basalt cliffs, up to 150 m. (500 feet) high and occupied mainly by Fulmars Fulmarus glacialis. At the base of the cliffs on the north and east coasts there are large areas of boulder scree. The west coasts are much less precipitous and the tops of the islands are covered with a patchwork of grassland, grazed by sheep, and bare rock.
In contrast Eilean Mhuire is composed of sedimentary rocks of the Lower Jurassic with dissected cliffs suitable for Guillemots Uria aalge and Kittiwakes Rissa tridactyla. The summit of Eilean Mhuire rises to 93 m. (307 feet) but most of the cliffs are no more than 66m. (200 feet) high. Seabird counting is therefore comparatively easy. The top of Eilean Mhuire provides excellent grazing, having few rocks.
Land mammals include sheep Ovis aries, black rats Rattus rattus, and otters Lutra lutra. The islands have remained uninhabited, save for visiting shepherds and natural history expeditions since 1901.
Although no formal census was made, it was possible, because of the limited area, to become familiar with the individual territories of the landbirds, especially the less numerous species. Figures quoted for landbirds are therefore based on casual, but sustained, observation.
The problems posed by seabirds vary according to species and are discussed below for the individual species. Where, for example, the figure 1,550 ±120 is given, this indicates that 1,550 was the mean of the observers' individual counts. The figure 120 is an estimate of the standard error of the mean based on the variation between the observers' counts. Counts were made only once in each year. All counting was done from land except where it was obvious that birds would be missed due to dead ground, in which case the count was made from the sea.
Figure 1 . Map of the Shiant Islands (Outer Hebrides). The letters indicate the sites of the main Puffin colonies (described in Bird Study, 19:1-6). Colony K on the Galtachean (inset) is half a mile southwest of the 'O' of Stocanish.
THE BREEDING SEABIRDS
Storm Petrel Hydrobates pelagicus.
No nests were found and no birds were heard calling at night, although a thorough search was not made. However, night mist-netting at a site on Eilean an Tighe yielded a total of 38 birds in four visits (one in 1970, three in 1971), and out of 20 birds specifically examined four had a well-developed brood patch. While the species may breed, the numbers are probably not large.
Manx Shearwater Puffinus puffinus.
Dr B. B. Roberts (pers. comm.) has heard the species calling at night over the Shiants. We noted no calling or dusk gatherings but saw the species at sea on five days in 1970 with a maximum day total of four on 24 June. In 1971 the species was only seen on the crossings from Tarbert, Harris at least 150 on 19 June and four on 3 July. As no thorough search was made at night the evidence is insufficient to determine whether or not the species breeds.
Fulmar Fulmarus glacialis. Several pairs were recorded on 8 June 1910, but were not proved to be nesting. Young were seen in 1926 and there were several hundred pairs by 1934 (Fisher 1952). The subsequent development of the colony is detailed in Table I. The 1970 count included pairs apparently occupying nest sites and was 3,781 ±132 pairs. Although the different counts are not strictly comparable due to observer bias and the presence on the cliffs of a varying and unknown proportion of non-breeding birds, it is probable that the colony is still increasing. In 1970 the first young hatched about 1 July and in 1971 the first chipping egg was seen on 29 June. Two different blue morph birds were seen in 1970, but it is not known if they were breeding.
TABLE I - NUMBER OF PAIRS OF FULMARS ON THE SHIANTS
The 1958 figures are probably unreliable due to the date of the count.
Shag Phalacrocorax aristotelis.
Nests were counted directly, except for a very few cases where they were inaccessible: the number of pairs was then estimated from the activity at the colony.
Table II compares counts made by the Westminster School Expedition (per D. Saunders) in 1958 and by the present expeditions. It is not certain whether the 1958 estimate of accuracy is a standard error, as it is in the present study.
TABLE II. NUMBER OF SHAG NESTS ON THE SHIANTS
The overall population may have decreased since 1958, and there has been a massive decline in nesting on Eilean Mhuire. This is unexpected since there is no lack of suitable nesting sites on that island. Conversely there has been some increase on Garbh Eilean and the largest colony, 220 ±20 pairs, is now found among boulders on the east coast of Garbh Eilean.
The main hatching period was the last week of June in 1970, but in 1971 a much higher proportion hatched before this time.
Great Black-backed Gull Larus marinus.
For this and the two following species, occupied nests and/or pairs of adults defending territory at the colony were counted.
Harvie-Brown and Buckley (1888) record the nesting of scattered pairs. By 1953 approximately 60 pairs nested on Eilean Mhuire, and the species was common on the other islands (Scott 1954). In 1970, there was a colony of 150 ±10 pairs nesting in grass on the very top of Eilean Mhuire where sheep graze, while there were colonies of 15 ±1 pairs at the southern tip of Eilean an Tighe, and 13 ±1 pairs at Stocanish. Scattered pairs were found elsewhere, making the breeding population 197 ±15 pairs. The population of Eilean Mhuire had possibly increased slightly by 1971.
Lesser Black-backed Gull Larus fuscus.
Harvie-Brown and Buckley (1888) recorded some 40 pairs near the southeast tip of Eilean Mhuire but Scott (1954) did not even see the species. In 1970 we found three pairs among the Herring Gulls at Stocanish and a further eight or nine pairs on the cliff top close to the summit of Eilean an Tighe. The latter colony had increased to about 16 pairs in 1971.
Herring Gull Larus argentatus.
The total 1970 breeding population was 204 ±20 pairs, nests being found on all islands including the Galtachean. Most were on the sloping rocky shores of the west coast of Garbh Eilean. The nests were rarely more than about 15 m. (50 feet) above sea level, except for a colony of some 35 pairs at about 45 m. (150 feet) in grass above the cliff top at Stocanish. No significant changes were noted in 1971.
Kittiwake Rissa trdiactyla
Individual occupied nests were counted on the cliff faces. In 1970. there were 1,000 ±15 nests of which the great majority were on Eilean Mhuire. Totals for other islands were: Garbh Eilean, 37; Eilean an Tighe, 95 ±1; Galtachean, 97.
The colony is one of the largest in the west of Scotland but the present count is 40% less than the 1,330 nests in 1958 (Coulson 1963). The Kittiwake is not a species for which one would expect substantial observer bias and so the present result is surprising in view of the fact that Coulson tentatively predicted a 33% increase in the decade 1959-69. The result might be explained to some extent by the fact that the 1958 count was made in late August, by which time additional sub-adult birds may have come ashore to occupy sites in and around the colony (Coulson 1959). Hatched young wore first noted on 4 July in 1970 and on 22 June in 1971.
Razorbill Alca torda
It was not practicable to count only those birds clearly associated with an egg or chick and all adults visible ashore in the colony were therefore counted. Where ringing in the centre of colonies enabled us to make a very approximate estimate of the number of successful breeding pairs, as revealed by eggs and chicks, it was found that one bird visible in the colony was roughly equivalent to one successful breeding pair. Any exact conversion factor would be dependent on the proportion of non-breeding birds. Indeed, personal observation of Razorbill colonies in late June and July suggests that many of the birds counted are not breeding; and, of the breeding birds, one parent may be tending the egg or chick and remain invisible to the observer, while the other is away fishing at sea. Counts of visible birds may then mean that the breeding population is estimated using, in part, non-breeding birds as an indicator of the breeding strength of the colony.
The Razorbill breeds on all islands and. in 1970, the total number of individuals counted was 3,535 ±240, the largest concentration being 1,150 ±90 along the east boulders of Garbh Eilean. In 1971 seven sample colonies, comprising about a quarter of the total population, were counted, in every case within two days of the 1970 date of counting. Changes from -52% to +280% were noted. Even excluding two colonies where less than 50 birds were counted in both years, changes from -52% to + 125% are found. Thus a considerable variation in the number of Razorbills visible according to time of day and weather conditions is suggested, unless a high proportion of Razorbills move their nesting sites from one year to the next. C. J. Bibby (pers. comm.), who spent an entire day observing the fluctuations in numbers at a Razorbill colony in Co. Clare, Eire, in June 1971, found that the maximum numbers visible were about three times the minimum. The overall change for the seven colonies was +4%, suggesting little change in the total population between 1970 and 1971.
In 1970, first departures were noted from about 26 June, and about one-third of the chicks had left by 8 July and two-thirds by 12 July.
Guillemot Uria aalge.
Individual adult birds ashore at the colony, not breeding pairs, were counted for the same reason as with the Razorbill. The relationship between the number counted and tile number of breeding pairs is unknown.
Southern (1962) considered the approximate size of the Shiants colony to be 3,000. The total 1970 count, including the Galtachean, was 7,970 ±350 birds. The majority of these were found on Eilean Mhuire, particularly the northeast cliffs. The largest colony outside Eilean Mhuire was at Stocanish where there were 1,440 ±60 birds.
Seven sample colonies, close to those used for the Razorbill, comprising nearly one-half of the total population, were counted in 1971. Changes from 0 to +135% were recorded. the overall change being + 12%. In fact six changes from 0 to + 18% were noted and only one small colony (20 birds in 1970; 57 in 1971) increased by so much as 135%. Between 1970 and 1971 the Guillemot population has probably increased by 10-15%. A newly colonised ledge was seen on Eilean Mhuire in 1971.
Compared with the Razorbill, the colony changes observed for the Guillemot show a smaller scatter, indicating that weather and time of day have less influence on counts. This is supported by the observations of C. J. Bibby (pers. comm.) in Co. Mayo, Eire, in June 1971. Watching a Guillemot colony throughout an entire day, he observed a 10% variation in numbers (c.f. c. 200% for Razorbill).
First departures occurred on about 30 June in 1970 and 26 June in 1971. In 1939, 11.1% out of 1,126 Guillemots were bridled. By 1949, 9.8% out of 581 were bridled, an insignificant decrease of 1.3% (Southern 1962). In 1970, 10.4% of 680 Guillemots were bridled.
Tystie Cepphus grylle.
The breeding population probably does not exceed ten pairs.
Puffin Fratercula arctica
BREEDING LANDBIRDS AND VISITORS
*Gannet Sula bassana.
Although not breeding, from one to 50 were observed offshore every day in both years. At least 80% of the birds were in full adult plumage.
*Grey Heron Ardea cinerea.
Two on 7 July 1970.
Eider Somateria mollissima.
Scott (1954) observed six pairs nesting on Eilean Mhuire. In 1970 we found eight to ten females with seven chicks and, in 1971, nine or ten females with 11 chicks.
*Red-breasted Merganser Mergus serrator.
One on 25 June 1970.
*Grey-lag Goose Anser anser.
One on the sea on 21 June 1970.
*Barnacle Goose Branta leucopsis.
One seen by us on 24 and 29 June 1971 was present for much of July (A. Pollard, pers. comm.).
*Buzzard Buteo buteo.
One on 28 and 29 June 1971.
White-tailed Eagle Haliacetus albicilla.
Harvie-Brown and Buckley (1888) recorded an eyrie in 1879 and 1887.
Golden Eagle Aquila chrysaetos.
Earlier this century, the species nested on the north cliffs of Garbh Eilean (W. A. J. Cunningham, in litt.)
During the 1970 visit, an immature Golden Eagle frequented the northern cliffs of Garbh Eilean. It was joined by an adult on 1 July, the latter staying until our departure. In 1971 two sub-adults were resident and a rudimentary eyrie could be seen on the north cliffs of Garbh Eilean. A third eagle arrived on 28 June but it is not certain if it stayed.
*Peregrine Falco peregri nus.
A single bird was seen on two days in both years. Three of the four records were certainly of immature birds.
Oystercatcher Haematopus ostralegus.
In 1970, about 15-20 pairs bred throughout the islands, including the Galtachean. There were no pairs on the Galtachean in 1971 when the total population was 13 ±1 pairs.
*Ringed Plover Charadrius hiaticula.
There were 11 on 24 June and one on 1 July 197 1.
*Turnstone Arenaria interpres.
Observed, in 1970 only, on four days between 22 June and 1 July, with a maximum day total of four on 22 June.
Snipe Capella gallinago.
In 1970, apparently three pairs on Eilean an Tighe and two on Garbh Eilean; in 1971, an increase to five or six pairs on Eilean an Tighe and two on Garbh Eilean. It was not seen on the much drier Eilean Mhuire.
*Redshank Tringa totanus.
Seen on six days during each visit, but never more than two birds on one day.
Common Sandpiper Tringa hypoleucos.
In 1970 there was a pair with at least one chick on the shore near the cottage. The species did not breed in 1971 and was only seen on three days with a maximum day total of three on 2 July.
*Curlew Numenius arquata.
Observed on seven days in 1970 with a maximum day total of seven on 7 July, and on six days in 1971 with a maximum day total of nine on 2 July.
*Arctic Skua Stercorarius Parasiticus.
Observed on nine days in 1970 with a maximum day total of seven on 7 July, and on 11 days in 1971 with a maximum day total of 11 on 27 June.
*Great Skua Catharacta skua.
One on 2 July and two on 3 and 6 July in 1970; one on 23 June 1971.
Common Gull Larus canus.
In 1970, neither nests nor young were found, but at the start of our stay two pairs were defending territory against human intrusion in the centre of Eilean an Tighe. This behaviour rapidly declined and was not observed in 1971, when the species was seen on all days except two, though never more than three birds on any day.
Rock Dove Columba livia.
At the most there are three or four pairs. One nest was found on Eilean Mhuire in 1970.
*Collared Dove Streptopelia decaocto.
Single birds were seen twice in 1970 and once in 1971.
*Swift Apus apus.
In 1970, a pair was seen with surprising regularity feeding, often close to the ground, above the eastern half of Garbh Eilean. The maximum day total was four on 29 June. Swifts were only seen on four days in 1971 with maxima of two on 25 June and 2 July.
Skylark Alauda arvensis.
Harvie-Brown did not report the species in 1879 but noted that it was 'far from uncommon' on Garbh Eilean and 'scarce or absent' on Eilean Mhuire on his visit in 1887. Scott (1954) recorded two pairs on Eilean Mhuire. We found between 10-15 pairs distributed among the three major islands.
*House Martin Delichon urbica.
Two were seen on 19th, and one on 21 and 25 June 1971.
Raven Corvus corax.
Scott (1954) thought a pair probably nested on Garbh Eilean. We noted two birds on several occasions in 1970 and six to eight individuals were present in 1971. No used nest was found.
Hooded Crow Corvus corone cornix.
Scott (1954) found three pairs on Eilean Mhuire, five on Garbh Eilean and four pairs on Eilean an Tighe. We recorded one or two pairs on each of the three major islands.
Wren Troglodytes troglodytes.
The species was numerous all around the coast but was absent from the Galtachean.
Song Thrush Turdus philomelos.
Although successful breeding apparently did not occur in 1970, two birds were heard singing from the basalt cliffs of Garbh Eilean. Harvie-Brown and Buckley (1888) also recorded a Song Thrush singing from the cliffs.
Blackbird Turdus merula.
In June 1967 L. R. Higgins (unpublished) saw a hen and three juveniles. A male was seen on three separate days in 1970. In 1971, certainly one and probably two males were in residence on Garbh Eilean, one male probably being paired, but no evidence of breeding was obtained.
Wheatear 0enanthe oenanthe.
In 1970, there were three pairs on Eilean an Tighe and three or four on Garbh Eilean, but none on Eilean Mhuire, possibly due to lack of suitable nesting sites. There were probably two fewer pairs in 1971.
*Sedge Warbler Acrocephalus schoenobaenus.
A singing male was heard on 22 June 1971.
Meadow Pipit Apithus pratensis.
The majority of the 15-20 pairs were on Garbh Eilean.
Rock Pipit Anthus spinoletta.
The Rock Pipit is more numerous than the Meadow Pipit, being found on all islands, including the Galtachean.
Starling Sturnus Vulgaris.
The total population, including juvenile flocks, was 100-200 birds. Most nests were in walls remaining from former habitations.
Twite Acanthis flavirostris.
There were one or two pairs on Eilean an Tighe and about five on Garbh Eilean in 1970: 1971 saw a slight increase to about three pairs on Eilean an Tighe and six or seven on Garbh Eilean. Although a pair was once seen on Eilean Mhuire in 1971, the species probably does not breed there, nor on the Galtachean.
*Birds marked with an asterisk are first published records for the Shiant Islands.
It is a great pleasure to be able to thank many people for their help, particularly N. Nicolson for permission to visit the Shiants; D. Macleod for provisioning us and arranging transport from Harris; C. J. Bibby and Drs W. R. P. Bourne and J. M. Boyd for comments on earlier drafts of this paper; the Seabird Group and especially D. Saunders for encouragement and financial support; Magdalene and Pembroke Colleges, Cambridge, for financial support; and, most of all, C. J. Mackenzie-Grieve, J. H. Marchant, H. A. Slatter and D. J. Steventon for their cheerful companionship on the Shiants and invaluable assistance in making the observations on which this paper is based.
The numbers and distribution of seabirds on the Shiants (Outer Hebrides) were estimated by Cambridge University parties in summer 1970 and 1971. A list of observations on the landbirds is included.