Puffins on St Kilda in 1972

The Puffin population on St Kilda declined dramatically between the early 1960's and 1971. A census of Hirta and Dun, together with some observations on Soay in 1972, did not indicate any marked continuing decline. Some redistribution of Puffin colonies and recolonisation of areas of Holcus grassland was observed. The validity of colony size assessment by counting birds is discussed.     

Repeated selection of morphometric traits in the Soay sheep on St Kilda

1. Long-term studies allow the outcomes of repeated selection events to be monitored. Here, we investigate phenotypic selection in successive winter mortality events in the Soay sheep of St Kilda, Scotland, between 1985 and 1996. Selection of three quantitative morphometric traits, body weight, hindleg length and incisor arcade breadth, was investigated in different sectors of the population.
2. Evidence from fitness differentials of positive directional selection for large size was repeatedly found in lambs and adult females. Selection in the opposing direction was only found in one year in lambs.
3. Selection gradients showed that in most years when significant selection occurred, body weight was the focus of direct selection, whereas selection of hindleg length and incisor breadth was indirect, arising from their correlation with body weight.
4. Selection was strongest in years of low over-winter survival and almost absent in years when survival was high. Intensity of selection was greatest in lambs, emphasizing the differences in selection pressure experienced by different sectors of the population, in addition to the temporal variation in selection pressure due to population density and environmental conditions.
5. Despite repeated positive selection of body weight, no evidence of a change in the population mean was found over the course of the study.     

Metazoan-protozoan parasite co-infections and host body weight in St Kilda Soay sheep

For hundreds of years, the unmanaged Soay sheep population on St Kilda has survived despite enduring presumably deleterious co-infections of helminth, protozoan and arthropod parasites and intermittent periods of starvation. Important parasite taxa in young Soay sheep are strongyles (Trichostrongylus axei, Trichostrongylus vitrinus and Teladorsagia circumcincta), coccidia (11 Eimeria species) and keds (Melophagus ovinus) and in older animals, Teladorsagia circumcincta. In this research, associations between the intensity of different parasite taxa were investigated. Secondly, the intensities of different parasite taxa were tested for associations with variation in host weight, which is itself a determinant of over-winter survival in the host population. In lambs, the intensity of strongyle eggs was positively correlated with that of Nematodirus spp. eggs, while in yearlings and adults strongyle eggs and coccidia oocysts were positively correlated. In lambs and yearlings, of the parasite taxa tested, only strongyle eggs were significantly and negatively associated with host weight. However, in adult hosts, strongyles and coccidia were independently and negatively associated with host weight. These results are consistent with the idea that strongyles and coccidia are exerting independent selection on Soay sheep.     

Modeling linkage disequilibrium in natural populations: the example of the Soay sheep population of St. Kilda, Scotland

The use of linkage disequilibrium to localize the genes underlying quantitative traits has received considerable attention in the livestock genetics community over the past few years. This has resulted in the investigation of linkage disequilibrium structures of several domestic livestock populations to assess their potential use in fine-mapping efforts. However, the linkage disequilibrium structure of free-living populations has been less well investigated. As the direct evaluation of linkage disequilibrium can be both time consuming and expensive the use of simulations that include as many aspects of population history as possible is advocated as an alternative. A simulation of the linkage disequilibrium structure of the Soay sheep population of St. Kilda, Scotland, is provided as an example. The simulated population showed significant decline of linkage disequilibrium with genetic distance and low levels of background linkage disequilibrium, indicating that the Soay sheep population is a viable resource for linkage disequilibrium fine mapping of quantitative trait loci.     

The Food of Nestling Jays and Magpies

Half of the 4,000 Grey Plovers which reach the Wash in late summer remain there to moult; more than a quarter are already in arrested wing moult, having begun elsewhere. The authors discuss this condition and also the birds' movements in the light of ringing recoveries.     

Ageing in a variable habitat: environmental stress affects senescence in parasite resistance in St Kilda Soay sheep

Despite widespread empirical evidence for a general deterioration in the majority of traits with advancing age, it is unclear whether the progress of senescence is chronologically determined, or whether factors such as environmental conditions experienced over the lifespan are more important. We explored the relative importance of ‘chronological’ and ‘environmental’ measures of age to changes in parasite resistance across the lifespan of free-living Soay sheep. Our results show that individuals experience an increase in parasite burden, as indicated by gastrointestinal helminth faecal egg count (FEC) with chronological age. However, chronological age fails to fully explain changes in FEC because a measure of environmental age, cumulative environmental stress, predicts an additional increase in FEC once chronological age has been accounted for. Additionally, we show that in females age-specific changes are dependent upon the environmental conditions experienced across individuals'' life histories: increases in FEC with age were greatest among individuals that had experienced the highest degree of stress. Our results illustrate that chronological age alone may not always correspond to biological age, particularly in variable environments. In these circumstances, measures of age that capture the cumulative stresses experienced by an individual may be useful for understanding the process of senescence.     

Grazing decisions of Soay sheep,Ovis aries,on St Kilda: a consequence of parasite distribution?

Relationships between the height of grass swards and herbage intake have been established for a number of large herbivores and sward height plays a major role in determining the selection of herbivore diets. However, tall grass swards also represent a more stable damp microclimate for gastrointestinal parasite larvae and tall swards contain generally greater numbers of parasites than short swards. Herbivores may then trade-off the need to maximise nutrient intake through the selection of tall swards with the need to avoid parasite ingestion. Stratified sward sampling techniques were used to determine the distribution of nutrients and parasites in a heterogeneous sward structure on the island of Hirta, St Kilda, which is grazed by a population of feral Soay sheep. The disparity between the short gap vegetation and the tall tussock vegetation was greatest in spring, when gap vegetation was some three-fold shorter than tussock vegetation; this led to tussocks offering greater nutrient and energy intake rate when compared to gap vegetation in spring. Parasites were concentrated in tussocks in spring, thus creating trade-offs. Such trade-offs were not present in summer when parasites were more evenly distributed across the gap/tussock sward structure and the nutritional advantages associated with grazing tussocks were diminished. The diet selection of the resident population of sheep was determined by recording the number of bites taken from gap and tussock vegetation by randomly selected focal animals over repeated 5-min grazing periods. Overall, all animals avoided grazing tussocks, which were most strongly avoided in the spring, and older animals avoided grazing tussocks to a greater extent. Overall, females with lambs and males avoided tussocks to a similar degree and both avoided them less than barren females. Faecal egg counts (FEC) of female sheep were negatively correlated with tussock selectivity and vice versa for male sheep. The interaction between the grazing behaviour of each sex and FEC is discussed in relation to the immunocompetence and nutrient requirements of the different sexes. The maximum disparity between the costs and benefits of the trade-off studied occurred in late winter/early spring which also represents the time of greatest nutrient demand in the Soay sheep, since many are close to starvation and/or are in the advanced stages of pregnancy. Grazing decisions of individuals at this time, determining relative nutrient and parasite intake, may then be related to subsequent fitness and survival.     

Land use history of Village Bay,Hirta, St Kilda World Heritage Site: A palynological investigation of plaggen soils

This paper presents findings based on a palynological investigation of artificially accreting (plaggen) soils from the settlement of Village Bay, Hirta, in the St Kilda archipelago, which was perhaps the most distant and inhospitable outpost of sustained human habitation in the British Isles. The soils were developed principally through the addition of turf ash and seabird waste, although some ash may have been derived from upland peats. It is assumed that the woodland pollen signal (much lower in the soils than in an upland peat site nearby) represents off-island sources. Corylus avellana-type pollen (frequent in upland sites), along with Potentilla-type, may provide markers in the Village Bay profiles for the addition of ashed hillside turf, and possibly peat, to the plaggen soils. Cereal-type pollen is well represented through the profiles and is often strongly associated with the record for Chrysanthemum segetum (corn marigold), a frequent indicator of arable land. The Brassicaceae signal may partly reflect the cultivation of cabbages; Chelidonium majus (greater celandine) may have been grown for medicinal use. Soil mixing has rendered radiocarbon dating meaningless at this site, but the establishment of a change in cultivation regime before AD 1830 may have been identified from the patterns of pollen concentration and preservation in the profiles.     

Diets and feeding of Fulmars Fulmarus glacialis during the breeding season:a comparison between St Kilda and Shetland colonies

Diets and nest attendance of Fulmars at Foula, Shetland and St Kilda, Outer Hebrides were examined during the breeding season. At Foula, sandeels formed the bulk of the diet, with fish offal and pelagic zooplankton of minor importance. At St Kilda, 71% of regurgitates consisted of pelagic zooplankton, probably captured at night. Dietary overlap between the two colonies was 14% by species composition. Foraging trips from Foula shortly after chick hatching generally lasted for less than 10 h, while trips from St Kilda late in chick rearing often lasted more than 24 h. The diurnal pattern of feeding suggested mainly nocturnal foraging from St Kilda but both diurnal and nocturnal foraging from Foula.     

Survival and behaviour of castrated Soay sheep (Ovis aries) in a feral island population on Hirta, St. Kilda, Scotland

The free-living population of Soay sheep on the island of Hirta, St. Kilda, in the Outer Hebrides, has been intensively studied since 1959. The present study was initiated to throw light on the causes of the high mortality rate of adult rams in comparison to that of ewes. In 1978, 1979 and 1980, a total of 72 male lambs was castrated within a day or two of birth. The survival of these castrates has been much longer than that of the entire rams, marked as controls, and longer than that of ewes of the cohorts of the same years. The daily activity pattern of the castrates was similar to that of ewes rather than that of rams. In particular, during the rut the castrates spent most of the daylight hours grazing, in contrast to the rams who were continuously moving and involved in agonistic and sexual encounters. This study substantiates the earlier assertion that the costs in energy of reproduction are a major cause of mortality in temperate zone ungulates. The social organization of some castrates was similar to that of females in that they remained with the home-range group of ewes into which they were born, but other individuals resembled males in that these castrates clubbed together in their own groups. These patterns of behaviour persisted throughout life.