Ecology, behaviour and predator-prey interactions of Great Skuas and Leach's Storm-petrels at St Kilda

• Main Author: Miles, William Thomas Stead
• Format: Thesis
• Language: English
• Published: 2010
• Subjects: QH301 Biology; QL Zoology; Q Science (General); Great skua
• Online Access: http://theses.gla.ac.uk/2297/; http://theses.gla.ac.uk/2297/1/2010MilesPhD.pdf; https://eleanor.lib.gla.ac.uk/record=b2842304

At the St Kilda archipelago, Outer Hebrides, declines have been recorded in the Leach's Storm-petrel breeding population, the largest in Britain and Ireland, and rapid increases in the population of Great Skuas. Leach's Storm-petrels have frequently been found in the diet of Great Skuas at St Kilda, where storm-petrels are active on land only at night and, unusually, skuas often hunt after dark. Apparent severe skua predation of Leach's Storm-petrels has raised conservation concerns regarding the sustainability of the St Kilda Leach's Storm-petrel population. However, it was recognised that this particular predator-prey relationship is a globally rare phenomenon, had not previously been studied for long at St Kilda (and never elsewhere), and warranted further research before conservation management interventions could be considered. Additionally, research on Leach's Storm-petrels was desirable in its own right, because the species had rarely been studied in the UK, due to its highly pelagic lifestyle and very remote breeding locations. The aim of this study was to increase our understanding of the ecology, behaviour and predator-prey interactions of Great Skuas and Leach's Storm-petrels at St Kilda. Results showed that Great Skua predation of Leach's Storm-petrels was considerable and sustained. Estimated numbers of Leach's Storm-petrels consumed annually by skuas were variable but averaged approximately 21,000 individuals per year. There was strong evidence from storm-petrel ringing and behavioural observations conducted at night that skuas fed predominantly on non-breeding Leach's Storm-petrels, which likely visit the archipelago in very large numbers each year, from huge colonies elsewhere, and probably play and important role in reducing impacts on the breeding population at St Kilda. It was found that Leach's Storm-petrels did not exhibit any specialised counter-predator adaptations to Great Skuas, and were very easily captured at night on the surface of the breeding colonies by skuas on foot. However, prey specialisation by skuas on nocturnally active seabirds (predominantly storm-petrels) did not create fitness advantages over prey specialisation on diurnally active seabirds or fish. Leach's Storm-petrel specialist skua pairs were very few and all pairs exhibited a tendency to feed on a diversity of prey and to switch prey-types between years. Adult and juvenile Leach's Storm-petrels were highly sensitive to light, and artificial light reduction measures in autumn helped prevent storm-petrel attractions and mortality in the village on Hirta. The St Kilda Great Skua population was found to be declining slightly, in contrast to the exponential growth recorded between 1990 and 2000, and Leach's Storm-petrel conservation issues now appear less severe than had been expected.