Grazing and vigilance by Soay sheep on Lundy island: Influence of group size, terrain and the distribution of vegetation

Animals allocate the time spent on different behaviours according to nutritional requirements, the distribution of food and the risk of predation. When the perceived predation risk is high, animals primarily behave in ways to increase their safety such as regularly scanning their surroundings, forming large groups and feeding in habitats with cover or high visibility. This study investigated which factors Soay sheep (Ovis aries) are most sensitive to in allocating time to different behaviours when the risk of predation is negligible. Continuous focal sampling was used to record the behaviour of the free-ranging Soay sheep on Lundy Island and measures of vegetation, group size and terrain were also recorded. Stepwise multiple regression produced a model of grazing with terrain and range of grass coverage as predictors and a model of vigilance with terrain as a predictor. Sheep grazed for longer on the slopes and as the range of grass cover decreased. They were more vigilant on the slopes. The results support the view that the group size effect largely depends on anti-predatory vigilance and suggest that time-budgets are more sensitive to other factors in the absence of predation.