Using inter-colony variation in demographic parameters to assess the impact of skua predation on seabird populations

Large skuas and gulls are top predators in marine ecosystems, feeding on shoaling fish, fishery discards and facultatively on smaller seabirds. As generalist predators they may have deleterious impacts on prey populations of seabirds, particularly when alternative foods are scarce. Declines in discards and lipid-rich shoaling fish may result in these large scavenging birds turning to prey on seabirds to meet their nutritional needs, yet we know relatively little about seabird predator-prey dynamics. Declines in Black-legged Kittiwakes Rissa tridactyla in the UK are attributed to reductions in Sandeel Ammodytes marinus availability, but may also be due to predation by Great Skuas Stercorarius skua in some parts of their range. We investigate whether variation in two demographic parameters (breeding success and population growth rate) of Kittiwake colonies across Shetland are explained by skua population density at increasing spatial scales (rings with radii of 0-5, 5-10, 10-15, 15-20, 20-25 and 25-30 km) and Kittiwake population density. These explanatory variables do not explain a significant amount of the variation in annual population growth rate (lambda), but our estimate of population change is highly conservative and we cannot exclude the possibility of type II errors. Kittiwake breeding success is positively correlated with Sandeel availability and negatively correlated with the number of Kittiwakes at the focal colony. Having controlled for these effects the number of Great Skuas also has an influence on breeding success, being negatively correlated at the scale of 5-10 and 20-25 km, but positively correlated at the scale of 10-15 km. In addition, analysis of Kittiwake populations subdivided into exposed or protected cliffs reveals that exposed sub-colonies declined more steeply than protected ones - presumably as a function of differences in susceptibility to Skua predation. We propose that comparing differences in demographic rates may be useful in unravelling seabird predator-prey dynamics, but only where there is a comprehensive demographic dataset, where it is possible to correct for confounding factors such as food availability, and information on habitat-predation interactions.