This thesis investigated kleptoparasitism in mixed-species foraging aggregations of gulls (Laridae). Kleptoparasitism, or food stealing, is a strategy used frequently by gulls. Gull populations are increasing in urban areas despite declining overall. Understanding the role of kleptoparasitism in aiding gulls to invade urban environments was a central aim of this research. A second aim was to develop a model of kleptoparasitism using a compartmental modelling approach from evolutionary game theory and to test this using real foraging data. Fieldwork was undertaken at two study sites: a coastal site (Brancaster beach, Norfolk, UK) and an urban site (Billingsgate Market, London, UK). The focal species were the Great black-backed gull (Larus marinus), Herring gull (Larus argentatus), Black-headed gull (Chroicocephalus ridibundus), and Common gull (Larus canus), these species forage together but differ in size and competitive ability. Foraging at the sites was recorded and analysed for kleptoparasitic incidents. Three kleptoparasitic strategies were considered: aggressive, stealth and scramble kleptoparasitism. Four studies were conducted: Study 1 investigated differences in the rate of kleptoparasitism between the study sites and assessed the ecological predictors of this difference. The results showed kleptoparasitism was higher at the urban site and higher population density was the best predictor of this. Kleptoparasitism may aid invasion and increase the range of environments a gull can tolerate by helping them meet their energy needs in novel environments. Study 2 described the patterns of kleptoparasitic behaviour observed at both sites. Large species used aggressive kleptoparasitism against smaller species, and smaller species used stealth kleptoparasitism when stealing from larger species. The use of stealth kleptoparasitism by smaller, subordinate foragers was identified as an empirical example of a Marauder strategy. Kleptoparasitic strategies were used flexibly to compete for resources against opponents of different competitive abilities. Study 3 examined what strategies, other than kleptoparasitism, subordinate foragers at Billingsgate used to compete for resources. Subordinate gulls foraged for longer, stayed closer to potential food locations, arrived first at patches and took more risks to obtain food than dominants. Study 4 developed a game-theoretical model and compared this model against the foraging data for Billingsgate. The results indicated the density of different foraging behaviours at Billingsgate may be at an equilibrium, but only 23% of foragers were using evolutionarily stable strategies. This result was attributed to a one-species model being used to describe a population containing three species of differing competitive ability. Further work applying game-theoretical models to field data is needed to assess how effectively gulls use kleptoparasitic strategies, particularly in novel environments such as urban areas.

The bioaccumulation of persistent organic pollutants (POPs) is of particular environmental concern in top predators, which accumulate high concentrations of POPs that can cause adverse effects. Previous small scale studies found high concentrations of POPs in adult great skuas, Stercorarius skua, a top predator in the marine environment. This thesis investigates the factors affecting concentrations and patterns of POPs (contribution of individual POPs to the ΣPOPs) in the great skua across its breeding range in the north-east Atlantic. Clear differences between colonies in both concentration and pattern of POPs in adult plasma were not indicative of being caused by long range transport of POPs in the atmosphere. Variation in diet between colonies is the mostly likely explanation for these colony differences, with great skuas from some colonies having a greater proportion of fish in their diet whilst others eat more seabird prey. Although seabirds are often used in studies of POPs in the environment, the effect of migratory behaviour has not previously been studied in detail. By using a combination of global location sensor (GLS) loggers and feather stable isotopes from winter grown feathers, the wintering areas of individuals from three breeding populations of great skuas were identified. Great skuas spend the winter in three distinct areas across the North Atlantic, with birds from the same breeding colonies wintering in different areas. In two of the three breeding populations, wintering area explained a significant proportion of variation in organochlorines (OCs) concentrations and pattern. However in the colony with the highest concentrations of OCs, no effect of wintering area was found, possibly as a result of these great skuas feeding at a higher trophic level during the breeding season than other populations. Temporally, concentrations of OCs were higher in 1980 than 2008 in eggs, whilst newer contaminants polybrominated diphenyl ethers (PBDEs) and perfluorinated chemicals (PFASs) show the opposite trend. In conclusion, concentrations of POPs in the great skua were influenced primarily by breeding season diet with wintering area and sex also having small but significant effects on POPs. Wintering area explained the most variation in the pattern of POPs in great skuas. The POP concentrations found in this study exceeded those which have been found to cause adverse effects on the immune system and reproduction in other species of seabird.