An empirical and theoretical investigation of kleptoparasitic foraging behaviour in mixed-species aggregations of gulls (Laridae)

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