Birds

Seabirds would have been the first avifauna to reach the new archipelago. Nowadays St Kilda is famous – and rightly so – for its huge seabird colonies but it also has an important function as a vital stop-over for many migrant and non-resident species.

Migrants

Northern fulmarSmall in size though it is, St Kilda is not as inaccessible as one might expect. Each year, in spring and in autumn, the islands are visited by an ever-growing list of migrants and vagrants. Some are on their regular route to and from more northerly breeding grounds, while others are blown off course; all find the archipelago a welcome stop-over in adverse weather. Over a hundred species may turn up in any year and since the first complete checklist of birds was compiled in 1978, 49 new species have been added to the list, about two a year on average.

Breeding land birds

Peregrine falcon chickThirty-three species of land birds (including waders and ducks) have bred at one time or another on St Kilda over the last 50 years. A few, such as the white-tailed eagle, peregrine falcon, corn crake, dunlin, rock dove, and perhaps twite, were probably regular breeders in the past; others –such as red-breasted merganser, dunlin, sky lark, song thrush, Eurasian tree sparrow and corn bunting may only have bred only sporadically. Some of these probably gave up altogether when the people left and cultivation ceased.

‘The many interesting birds seen and collected by George Stout and Eagle Clarke in autumn 1910 and 1911 gave St Kilda a reputation as a good place to see rare birds. However, regular observations, starting with the restoration of a resident human population on the island in 1957, showed that these pioneering ornithologists had been very lucky and those months must be regarded as exceptional. Atotal of 228 migratory species (including some like the northern wheatear and meadow pipit, which also breed) have been recorded, but of these 54 only two to five times and 45 only once. Even in the best year for variety (2000), only 140 species, including breeding birds, were recorded.
Atlantic puffins over village bayOwing to the isolated position of the islands, the only regular migrants are the relatively few species that nest in Iceland and Greenland. In spring northern wheatear, meadow pipit, white wagtail and redwing are common, and there are usually a few ducks, geese, swans, merlin, dunlin, common redshank and mew gulls. In autumn, the same four passerines are common, and there are usually a few ringed plover, red knot, dunlin, sanderling and ruff. Most other species can be regarded as lost individuals, either overshooting mainland Britain during their spring migration (e.g. hirundines, warblers, chats, spotted flycatcher) or by being drifted westwards towards the open Atlantic by easterly winds with overcast skies in the autumn (e.g. willow warbler, pied flycatcher and the less common warblers). The 21 American species were obviously completely lost, but it is remarkable that so many have found their way to the island, and there are now five or more records for Baird’s sandpiper, pectoral sandpiper and American golden plover. Other species will presumably be added to the list in the future . . . but there are unlikely to be any other regular land birds. More information is needed on seabird migration. Although shearwater passage has been noted in some springs, the offshore movements of species such as skuas and terns is unknown and they could be regular visitors, particularly in autumn.’ (Stuart Murray (2002): Birds of St Kilda. Scottish Birds supplement to Vol 23)


Table 3.1 Breeding waders/ducks and land birds based on the last 50 years' records

 
 
Annual Irregular Bred in earlier years
Waders/Ducks Eurasian oystercatcher
common snipe
common eider
common redshank
common teal
European golden plover
whimbrel
red-necked phalarope

 
Land birds common raven
hooded crow
St Kilda wren
northern wheatear
meadow pipit
rock pipit
common starling
common kestrel
peregrine falcon
sky lark
house martin
yellow wagtail
pied wagtail
stonechat
Eurasian tree sparrow
white-tailed eagle
corn crake
rock pigeon twite
song thrush
snow bunting
corn bunting
 
     

 


Table 3.1 Breeding waders/ducks and land birds based on the last 50 years' records

 
 Species
Status
Eurasian oystercatcher

About 50 pairs bred up to 1993, now numbers have almost halved perhaps due to predation by great skuas.
Common snipe Numbers have increased from the low in the 1930s although they still fluctuate annually. Up to 100 pairs may breed with the main concentrations in Village Bay, Glen Mor and on Dun.

Common eider

About 50 pairs breed each year on Hirta, Dun and Boreray.

Common starling

Up to 300 pairs breed each year, mostly on Hirta and Dun but flocks are also seen on Boreray and Soay.

Rock pipit

Most recent estimates are 70 pairs on Hirta (1993) and 17 pairs on Boreray (1980). Also a pair has been recorded on Stac an Armin in recent years.

Meadow pipit

Up to 20 pairs breed on Hirta, also known to breed on Soay and Boreray.

Northern wheatear

Between 30 and 60 pairs have been counted on Hirta mainly around Village Bay. Also a couple of pairs on Boreray.

Hooded crow

Several pairs likely to breed on Hirta, Boreray and Soay but nests seldom found.

Common raven

Several pairs likely to breed on Hirta, Boreray and Soay but nests seldom found.

St Kilda wren

Numbers vary but probably total around 230 pairs, with 100 on Hirta, about 50 each on Boreray and Soay, 25 on Dun and 2-3 pairs on Stac an Armin.

 
     

 

The St Kilda Wren
Perhaps the most notable land bird resident is St Kilda’s own distinctive subspecies of wren. Martin Martin mentioned them in 1698 and, in 1758, the Rev. Kenneth Macaulay speculated ‘how these little birds . . . could have flown thither or whether they went accidentally in boats’. Almost certainly they flew and distinctive subspecies of Troglodytes troglodytes have arisen on offshore islands elsewhere in the North Atlantic. In 1884 Seebohm first described the St Kilda wren as a new species but only a couple of years later it was reduced to the subspecific status it still enjoys to this day. Troglodytes troglodytes hirtensis is 2 or 3g heavier than mainland wrens, longer winged with long, thicker bill and legs. It is paler and more barred in appearance, has a slightly different song and lays larger, heavier eggs. Such adaptations may enable it to withstand the harsh, exposed conditions it has experienced on St Kilda, or else they might be the result of a limited gene pool amongst the original colonisers. All this has been achieved within 10,000 years, since the end of the last glaciation allowed plants and animals to return and recolonise St Kilda.


St Kilda WrenInitially the St Kilda wren attracted such attention from naturalists, museums and collectors that the islanders used to supplement their meagre income by selling skins and eggs. Fears were expressed that the subspecies might become extinct so in 1904, a special Act of Parliament was passed to protect both the St Kilda wren and another rare seabird nesting there, the Leach’s storm-petrel. Numbers may indeed have declined around the Village but the wren was never really in any danger of extinction since it was more abundant on the steep, dangerous cliffs around St Kilda’s coast. Numbers may vary slightly from year to year but probably total around 230 pairs, with over 100 on Hirta, about 50 each on Boreray and Soay, another 25 or so on Dun and even two or three pairs near the summit of Stac an Armin.

The wren’s distribution is closely linked to that of the Atlantic puffin and the highest densities are to be found on Dun and at Carn Mor on Hirta. A few have even been found on Stac an Armin. Sandeels dropped by Atlantic puffins, carcasses from adults predated by gulls and dead chicks at the mouth of nest burrows all support an abundance of carrion-eating insects upon which the wrens feed. The birds that manage to live around the Village probably benefited from the middens and food stores around the houses (and nowadays the MoD Base) and from rotting sheep carcasses in the cleitean.

Rock crevices, scree slopes, dry-stone walls, buildings and cleitean all provide shelter and ideal nesting sites for the wrens. On St Kilda the wrens have no real predators and, although their numbers may fluctuate from year to year due, presumably, to vagaries in the weather and food supply, the population of St Kilda wren seems secure.

Breeding Seabirds

The four main islands and associated sea stacks provide a variety of breeding habitat for the seabird community. The oceanic vegetation communities offer ideal nesting opportunities for burrow-nesting species such as Atlantic puffins and Manx shearwaters while the rocky cliffs contain extensive ledges for breeding common guillemots and razorbills. Rock crevices, including those on human-made structures such as houses and cleitean also provide nesting habitat for storm-petrels.

Great skuaSt Kilda’s exposed cliffs have long hosted seabird communities of historical fascination as well as exceptional biological importance. The northern fulmar population has existed here from time uncertain and was the only known British breeding site of the species until 1878; the first British (as recently as 1818), and only the third ever, specimen of Leach’s storm-petrel was collected on St Kilda; and, of course, St Kilda is famous as one of the best-known haunts of the now extinct great auk, being one of only three sites in the eastern Atlantic where this species is known certainly to have bred. That the ornithological history of the islands has been so well-documented testifies to their importance in a global context.

Ever since St Kilda was settled the seabirds have played a key role in the life of the human residents. In most places where humans and birds co-exist the relationship between the two tends to be one of exploitation, whether this be hunting, husbandry, or harvesting, and St Kilda is no different. Rarely, however, have humans depended on birds to the degree that the St Kildans depended on seabirds. During the long period of human habitation several species were captured for food, oil and feathers; their eggs were also eaten as well as being supplied to collectors. Although such exploitation was heavy it was also sustainable because the bird populations were so large.

 


Table 3.3: Population sizes of each species of seabird breeding on St Kilda in 1999-2000 and the relative importance of these in the Great Britain & Ireland and north-east Atlantic contexts.

 

Species
Population size1 Proportion of Great Britain and Ireland population
(%)2
Proportion of north-east Atlantic population
(%)3

Population trend on St Kilda
Northern fulmar 66,942

13.26 3.69
Stable after prolonged period of growth in the 20th century
Manx shearwater 4,803

1.35 1.26 Unknown
European storm-petrel 1,121

0.95 0.30 Unknown
Leach’s storm-petrel 45,433

92.10 89.29 Unknown
Northern gannet 60,428

24.14 23.64 Stable after prolonged period of growth in the 20th century
European shag 19

0.07 0.04 Probably little change since 1970s
Arctic skua 1

0.07 0.00 First bred 1999?
Great skua 169

1.47 1.38 Steady increase since colonisation

Mew gull

1 0.00 0.00 Very small numbers only since first records
Lesser black-backed gull

30 0.02 0.01 Decline from small numbers over latter part of 20th century
Herring gull

34 0.01 0.01 Decline from small numbers over latter part of 20th century
Great black-backed gull

32 0.12 0.04 Decline from small numbers over latter part of 20th century
Black-legged kittiwake

3,886 0.97 0.24 Decline over last three decades
Common guillemot

23,378 1.55 1.17 Stable over last three decades
Razorbill

2,521 1.28 0.89 30% decline over last 25 years
Black guillemot c. 25 0.10 0.02 Little change over last 25 years
Atlantic puffin 135,752 30.29 4.41 Some evidence of decline over last 25 years but insufficient data

 
 


1 The survey sampling unit for each of these species varies: for northern fulmar, Manx shearwater, European storm-petrel, Leach’s storm-petrel, northern gannet and Atlantic puffin the count unit is number of Apparently Occupied Sites/Burrows; for European shag, the gulls, including black-legged kittiwake, the count unit is number of Apparently Occupied Nests; for the skuas the count unit is number of Apparently Occupied Territories; and for common guillemot and razorbill the count unit is number of individual birds counted on breeding ledges.
2 Great Britain and Ireland includes United Kingdom, Republic of Ireland, Isle of Man and Channel Islands.
3 North-east Atlantic region includes Faroe Islands and Svalbard but excludes Iceland and Greenland.
 
 


The spectacle and clamour of one million birds at the height of their breeding season in north-west Europe’s largest seabird colony is startling evidence of the significance of St Kilda as a key component of the north Atlantic ecosystem. Here in the most numerous colony in the world, tens of thousands of northern gannets nest in high-rise communities on Boreray, Stac Lee and Stac an Armin; here, in the biggest colony in western Europe, nest more than 100,000 northern fulmars. More than 250,000 Atlantic puffins breed on the archipelago’s grassy slopes – the largest colony in Great Britain and Ireland. Not only during the day is seabird activity testament to the islands’ preeminence; St Kilda holds noteworthy populations of all three species of nocturnal petrels that breed in the north-east Atlantic, including the largest colony of Leach’s storm-petrel.

Leach’s storm-petrels are now surveyed using recently developed tape playback techniques. This entails playing recordings of the male’s chatter call in suitable habitat during their incubation period in order to elicit a response from incubating adults. Birds respond only to calls of the same sex; consequently not all individuals respond. It is therefore necessary to determine the proportion of incubating birds that respond to taped calls in order to obtain an accurate estimate of total breeding numbers. This is achieved by repeatedly visiting a section of a colony (calibration plot) on successive days and noting each new response. Visits are repeated until no new responses are elicited; the results are then analysed to determine response rate and as a result an estimate of the number of Apparently Occupied Sites (AOS). Applying this technique on St Kilda in 1999 yielded a population estimate for Leach’s storm-petrel of 45,433 AOSs (95% CL= 34,310-61,398).


Northern gannetOf the species comprising this outstanding seabird assemblage, the northern fulmar is perhaps the best known historically. An extremely long-lived species the northern fulmar is a member of the petrel family, the Procellariidae. Although well-known as scavengers at fishing vessels, St Kilda northern fulmars are actually less dependent on this method of foraging than birds from populations in other parts of the north-east Atlantic. They are present for the whole summer on the archipelago, raising one chick. An inhabitant of St Kilda for at least 1,200 years this is a species whose population has long been the focus of interest on the islands, having seen spectacular growth over the course of the 20th century. That the population remains in such good health is testimony to the productivity of the northern fulmar’s natural feeding areas in the waters around St Kilda.

A gregarious seabird, the Manx shearwater feeds from the surface of the sea by plunge-diving to modest depths to take a variety of small fish, cephalopods (e.g. squid) and crustaceans. Each pair raises one chick per year, visiting by night their nesting burrows in the grassy slopes of the islands.

 

St Kilda is especially important as the major breeding station of Leach’s storm-petrel in the north-east Atlantic. A nocturnal seabird that comes ashore only to nest in burrows, this petrel is accorded special protection by inclusion on Annex I of the EU Directive on the Conservation of Wild Birds; it is one of the species for which St Kilda has been classified as a Special Protection Area. Such is the difficulty of censusing this species that upper and lower estimates of its population size in Britain even in the last two decades have differed by an order of magnitude. Recently more precise estimates of the population numbers have been possible and as many as 45,000 Apparently Occupied Breeding Sites were identified in 1999.

Albeit nesting in lower numbers (i.e. 1,121 Apparently Occupied Breeding Sites), the closely related European storm-petrel also breeds in important numbers on St Kilda. This dainty, nocturnal petrel breeds in burrows and rock crevices (including those of the ruined houses and cleitean of St Kilda). Both storm-petrels feed mainly on zooplankton in the open ocean and are able to locate feeding grounds using olfaction.

The northern gannet is a large, brilliantly white bird that plunges from great heights to catch large fish such as herring and mackerel. On entry to the sea the birds might attain speeds of 100km/hr, reaching depths of up to c.15m. The northern gannet nests on Boreray and its satellite stacks Stac an Armin and Stac Lee. On these cliffs they raise one chick per year. In common with the northern fulmar, the breeding northern gannet population also increased markedly in the 20th century; the St Kilda colony is the largest in the world.

The great skua population is also of importance. Predation by great skuas on the storm-petrel population of the islands appears to have increased in recent years, perhaps as a consequence of changing fisheries practices at sea, but this fearless predator remains a key species in the seabird assemblage of St Kilda.

St Kilda has become inextricably identified with the Atlantic puffin; unsurprisingly as it is host to the largest breeding concentration in Great Britain and Ireland, mainly on Soay and Boreray. Atlantic puffins are members of the auk family; they dive from the surface of the sea to relatively shallow depths to catch their prey, which consists primarily of a variety of small fish. Both sexes assume a brightly-coloured bill during the breeding season that indicates their age and breeding status. They nest in burrows, rearing one chick per year. Population size has remained mostly stable on St Kilda over the last three decades.

Fledgling Atlantic puffins on St Kilda

The Atlantic puffin colony on Dun is one of the largest breeding concentrations in Britain. Access to the island is difficult at the best of times but, following on from detailed studies there in the 1970s, a measure of annual breeding success can be gained from a rather unfortunate happenstance across Village Bay. Although all unnecessary lights are extinguished, young puffins fledging from their burrows at the end of the breeding season are attracted into the MoD Base each night. The warden and any volunteer helpers patrol the area nightly for several weeks at the end of July/early August; the numbers of birds collected each year can range from 18 in 1985 to 1,409 in 1978. Each bird is weighed, ringed and released off the end of the pier. The numbers and body weight of the stranded fledglings caught each year correlates well with growth rates and annual breeding success back on Dun. The data are providing valuable long-term information on the fortunes of St Kilda’s Atlantic puffins.


St Kilda also hosts important breeding populations of the closely-related common guillemot. A cliff-nesting auk that also feeds on small fish such as sandeels, herring and sprat, the common guillemot dives from the sea surface to greater depths than the Atlantic puffin; it has been recorded at a depth of 180m. One chick per year is produced and, still flightless, is taken to sea by the male parent at the end of the breeding season.

A third auk that breeds in significant numbers on St Kilda is the razorbill. With a diet and foraging ecology that are very similar to the common guillemot, the two species are often seen in the company of each other at sea and are important indicators of the health of both the sea surface and the water column over the north-west European continental shelf. Razorbills breed in close proximity to common guillemots on the high cliffs of the archipelago, again rearing one chick per year.

Great auk
St Kilda is without doubt the most famous site in Britain for the now extinct great auk. Its steep cliffs may not have offered many nesting opportunities for this flightless seabird – the original ‘penguin’ – but several interesting eye-witness accounts survive. Perhaps the most informative is that of Martin Martin after his visit to St Kilda in 1697: ‘it stands stately, its whole body erected, its wings short, flies not at all, lays its egg upon the bare rock which, if taken away, she lays no more for that year … it comes without regard to any wind, appears the first of May and goes away about the middle of June' (In the modern calendar these dates would be two weeks later). Another account related how the great auks or garefowl ‘are taken by surprising them where they sleep, or by intercepting their way to the sea and knocking them on the head with a staff: they lay their eggs a little above the sea mark on rocks of easy access; they carry off their young soon to feed them at sea.’ But by this time (around 1758) the bird was a rare visitor to St Kilda. One was caught by the islanders in 1821.

Great Auk

Another was apparently caught on Stac an Armin around 1840 and, sadly, was one of the very last birds in existence. The great auk officially became extinct when a lone survivor was killed in Iceland in 1844.


Although at lower densities, other species also contribute to this dazzling assemblage. Not only is this a rich diversity of species but also a diversity of form and of function; an array of ecological and behavioural processes are represented here evidenced by a wide variety of feeding techniques and feeding loci in the sea. The black-legged kittiwake is a small gull whose breeding performance depends largely on its key food source of sandeels, and as such is an important indicator of the availability of these at or near the surface of the sea (as the auks would be at greater depths). Around 3,900 pairs of black-legged kittiwakes breed on the islands; just less than those that would indicate national importance. Other species that comprise the diverse spectrum of seabirds on the islands are four other gull species, the great black-backed, the lesser black-backed, the herring and the mew gull, all of which rely on more natural food sources at St Kilda than their conspecifics at less remote locations. Also present are the European shag and another auk, the black guillemot, both of which exploit inshore feeding habitats. Completing the assemblage is the Arctic skua, a species that relies for food by parasitising other seabirds. Not only has St Kilda been classified as a EU Special Protection Area for its important breeding populations of both storm-petrels but it is also officially recognised as such for the whole breeding seabird assemblage.