The Puffin Population of the Shiant Islands
By M. de L. Brooke
One of the biggest puffinries around British coasts is situated on the Shiant Islands, off Lewis, O.H. Assessing the size of vast colonies of this kind is a daunting problem, and the author describes the methods he adopted during a visit to the Shiants to obtain a reasonably reliable census.
The SHIANT ISLANDS, shown in Figure 1, lie in the Minch approximately five miles southeast of Lewis and 13 miles north of Skye. From 20 June to 8 July 1970 and from 19 June to 3 July 1971 they were visited by parties from Cambridge University whose aim was to study sea-birds of the Shiants, in particular the Puffin Fratercula arctica.
After visiting the Shiants in 1879 Harvie-Brown 'thought that the Puffin population was as big as that on St Kilda. He found Puffins all around the cliff-tops as well as on the basalt cliffs and amongst the boulders at the base of the cliffs on the northeast and cast sides of Garbh Eilean. On his second visit in 1887, Harvie-Brown noted that the whole circumference of Eilean Mhuire was densely colonised, probably at a greater density than on St Kilda, although the area colonised was perhaps smaller (Harvie-Brown and Buckley 1888). Lockley (1953) states that, following Harvie-Brown, 'later visitors to the Shiants have found it much less numerous'.
Nevertheless, after spending a day on the Shiants in 1947, James Fisher (per D. Saunders) thought that the colony was the largest in Britain after St Kilda and Foula, being as big or bigger than Clo Mor in Sutherland. Our results for 1970 indicate that the colony is still the third or fourth largest in Britain (D. Saunders in litt.), although, as can be seen from Figure 1, there is no doubt that the densely colonised area is now much smaller than it was in the late nineteenth century, and that the population has declined considerably. Local fishermen comment that the Puffin population has fallen noticeably even in the past few years.
N. Nicolson (in litt.), owner of the Shiants since 1937, did not notice a decline until about 1959, although Lockley's statement suggests that the decline may have begun much earlier. Dr. B. B. Roberts (pers. comm.) who has visited the Shiants on several occasions during the past 30 or 40 years. thinks that, in this period, a decrease of roughly 90% may have occurred.
The number of Puffins visible at a colony varies greatly from day to day and during the day, making any counts of such birds extremely difficult to interpret in terms of numbers of breeding pairs. The extent of this variation was studied in 1971 by marking out by string (to which the Puffins did not react in any visible way) a 1,000 square metres sample area of Colony G on a grazed steep grassy slope. The number of apparently occupied burrows within this area was studied independently by three observers using method (i) (see below) and figures of 366, 295 and 345 apparently occupied burrows resulted. A fourth observer who counted individually every burrow found 354 apparently occupied burrows.
As an alternative approach, the author threw a one square metre quadrat at random a hundred times within the sample area, the presence or absence of a burrow being recorded at each throw. The number of occasions on which a burrow was present was then multiplied by 10. There were found to be 430 occupied burrows. This result might be higher than those obtained by other methods due to biassed throwing, the thrower tending to ’aim’ at a burrow. Since the number of burrows was estimated in three different ways, to present a mean would be misleading. For present purposes there are taken to be 350+ 30 apparently occupied burrows within the sample area: 430 is not significantly different from this figure.
Counts of Puffins ashore (but not in the air) were made thrice daily as near as possible to 09.30, 13.30 and 20.30 hours B.S.T. All counts were within 40 minutes of the stated time and 77% were within 20 minutes. The results are presented in Table I.
*The presence of gulls in or close to the sample area may have influenced the count.
The colony slope is well sheltered from all winds from east, through south, to west. It is difficult to come to any conclusion about the influence of weather on counts although there is a suggestion that numbers are low when the wind strength is low, possibly because the sea is then more suitable for fishing.
The time of the evening count, 20.30 hours, was chosen to coincide with an expected period of high numbers. In fact, only on the last three days of counting did the numbers counted at 20.30 exceed those counted at 13.30 hours. Possibly a different picture would have emerged if some account had been taken of those Puffins circling the colony in the air, as well as those counted on the ground. Counts at 09.30 hours were consistently low.
On only one day, out of 13, did the number of Puffins exceed one-half of the total number of burrows.
In view of the evident difficulty of using direct counts to assess the population in absolute terms, the following four methods were employed and are discussed below. The basic unit used in the estimation of population is the apparently occupied burrow. It must be emphasised that methods (ii), (iii) and (iv) are essentially comparisons with method (i), and therefore their accuracy depends on the accuracy of method (i).
Method (i). Where the terrain was suitable, apparently occupied burrows were counted one pace (the pace being more versatile than the tape measure) to either side of a straight-line transect following the line of greatest slope up the colony. After several transects, the mean number of burrows in a strip one pace wide may be determined. Knowing the horizontal width of the colony, also paced, the total number of apparently occupied burrows is calculated.
Method (ii). The number of birds departing from burrows in the colony in unit time may be compared at the same time on the same day with the number departing from burrows in a colony of known size as determined by method (i). Method (ii) assumes that the rate of departure is directly proportional to colony size and is probably only of value when the colonies being compared are of roughly similar size, for the proportion of burrow departures missed by the observer would be expected to increase as colony size increases.
Method (iii). The number of birds standing at the colony may be compared simultaneously with the number at a colony of known size as determined by method (i). It is assumed that non-breeders and failed breeders are present in direct proportion to colony size. It is probably important that the wind should strike the two colonies at a similar angle. Method (iii) was particularly used for colonies among boulders, where it is possible that birds were hidden from view, although they were effectively standing at the colony. This would lead to an underestimation of numbers.
Method (iv). The colony size is estimated roughly from the area of the colony and the amount of activity there. This is crude and was only used for the smaller colonies.
Results for 1970 and 1971 for the colonies shown in Figure 1 are given in Table II. The percentage change between the two years is only given for colonies counted by method (i), the most accurate counting method. Colonies D and G were both counted in two distinct parts.
Adding the 1970 figures and including some small groups of burrows, there were, in that year, thought to be 77.000 ± 13,500 occupied Puffin burrows on the Shiants.
Between 1970 and 1971, changes from -51% to +50% were observed in the eight colonies counted by method (i), but six of these showed decreases of between 10% and 30%. The mean change for the eight colonies is -22%. Although all method (i) counts were done by the author, it is not impossible that some part of this decline could be due to an unconscious change in the criteria used to decide upon an occupied rather than an unoccupied burrow. Nevertheless, between 1970 and 1971, there may have been a decrease of about 20% in the Puffin population of the Shiants.
Figure 1. Map of the Shiants. The letters mark Puffin colonies described in the text. Colony K on the Galtachean (see inset) is half-mile southwest of the 'O' of Stocanish.
* The fringes of this colony are difficult of access and the increased number of burrows might be due to increased determination on the part of the author in 1971.
** No satisfactory count was achieved in 1971 but the colony was still the largest on the Shiants and, as in 1970, probably had two to three times the total number of burrows Colonies G and H; that is, very approximately, 30.000 occupied burrows.
***All method (iii) counts were repeated at least twice.
ASPECTS OF BURROW DENSITY
In two 3-metre-wide vertical transects, one central and one peripheral, in Colony A in 1971, the burrow density was found to be 0.41 and 0.08 occupied burrows per square metre respectively. The lower density towards the edge of the colony is not unexpected, particularly in a declining colony. It would seem that as a colony declines, so first there is a reduction in burrow density towards the edge and then later a contraction in the area colonised. No reduction in the area colonised was noted for any Shiants colony between 1970 and 1971.
If the pattern of colony decline just sketched is correct, then the ratio of unoccupied to occupied burrows would be higher towards the edge of the colony. This was observed in Colony A, being 4.42:1 in the central transect and 3.33:1 in the peripheral transect. In a centrally situated transect in Colony G the ratio of unoccupied to occupied burrows was 1.11:1. Such figures may not be used to estimate the extent of the decline of a Puffin colony since unoccupied burrows will disappear after prolonged disuse and newly breeding birds may dig new burrows rather than occupy the vacated burrows of dead birds.
FACTORS AFFECTING THE PUFFIN POPULATION
In 1970 there were, on the Shiants, close to 200 pairs each of Herring Gulls Larus argentatus and Great Black-backed Gulls L. rnarinus. Both species steal the food being carried by Puffins to their chicks, but only Great Black-backed Gulls were observed to kill and eat adult Puffins. Of the 200 pairs of Great Black-backed Gulls, only about 50 take adult Puffins, as shown by the presence of Puffin carcases in their territories. During July 1971 the mean rate at which Puffin carcases were brought to ten Great Black-backed Gull territories on Dun, St Kilda, was found to be 0.78 Puffins per territory per day (pers. obs,). The number of Puffin carcases on the Shiants suggests a lower rate of predation than on Dun. If 50 pairs of Great Black-backed Gulls kill adult Puffins at the rate of 0.5 Puffins per pair per day, then during the Puffins' breeding season of roughly 120 days, when they are most at risk from predation by gulls, some 3,000 adult Puffins may be killed. Such a figure, though speculative, is not so high as to suggest that predation by gulls is the sole factor responsible for the decline of the Puffin on the Shiants.
Rats, both black and brown, Rattus rattus and R. norvegicus, are found on the Shiants, having reached the islands in 1900 following a shipwreck on the Galtachean (N. Nicolson in litt.). Drawing on evidence from other Puffin colonies, such as Ailsa Craig, Lockley (1953) implicates rats in the decline of Puffins on the Shiants. In view of uncertainty as to when the decline began, this cannot be considered proven. We can offer no observations on interactions between rats and Puffins.
There has been a steep decline in the Puffin Fratercula arctica population of the Shiant Islands, Outer Hebrides, during the present century, though when this began is uncertain. Counting methods developed by Cambridge University parties in the summers of 1970 and 1971 are described and the results suggest a 20% drop in the population between the two years. The reasons for the decline are not known.
HARVIE-BROWN, J. A. & T. E. BUCKLEY. 1888 A Vertebrate fauna of the Outer Hebrides. Edinburgh.
LOCKLEY, R. M. 1953. Puffins. London.
M. de L. Brooke, Gadley House, Manchester Road, Buxton, Derbyshire.