A new study, published in the British Ornithologists’ Union (BOU) journal Ibis, has found that urban gulls are more effective at stealing food from their cohorts than their rural counterparts.
Food stealing, an exploitative process known as kleptoparasitism, is regularly employed by various gull species in order to acquire food at a reduced effort. Cutting expended energy costs in the foraging cycle is clearly beneficial to any organism, and exploiting food discovered through another individual’s effort makes for an easy meal for gulls.
Populations of British gull species have shown significant declines over the past half century and a number of these are now a conservation priority in the UK. These include European Herring, Great Black-backed, Common and Black-headed Gulls, which were the study species for the research. These four species are all now Red- and Amber-listed as Species of Conservation Concern. Despite their overall downward trends, the populations of some of these species have been increasing in urban areas, hence an investigation into the behaviours that are allowing these populations to thrive is critical to understanding how they might overcome recent declines elsewhere.
The researchers speculated that kleptoparasitism may support invasive behaviour by gulls by providing an effective way for them to meet their energy requirements when faced with changed environmental conditions. In light of this, they hypothesised that kleptoparasitism would occur at a higher rate in urban gull populations than in those in more traditional coastal foraging environments. Additionally, they sought to identify which ecological variables best predicted any difference in the rate of kleptoparasitism seen between these environments.
Fieldwork was conducted to record kleptoparasitic behaviour in mixed-species foraging flocks of gulls over a year (the kleptoparasitic interactions of interest were gulls stealing from other gulls). This research was conducted at two study sites: Brancaster Beach, Norfolk, and Billingsgate Market, London (a commercial seafood market).
The rate of kleptoparasitism was found to be significantly higher at the urban site, with a higher gull population density and larger prey size the best predictors of this. The results suggest that the behaviour may aid invasion and increase the range of environments a gull can tolerate by helping them meet their energy needs in novel environments where normal foraging behaviours are difficult to implement. Kleptoparasitic strategies appear most effective when individuals are forced to forage together at higher densities and food items are large enough to provide conspicuous visual cues highlighting the presence of food.
European Herring Gulls are now faring much better in urban environments compared to their rural counterparts, this aided considerably by scavenging on human waste (Tom Kennedy).
Spencer R, Russell Y I, Dickins B J A & Dickins T E. Kleptoparasitism in gulls Laridae at an urban and a coastal foraging environment: an assessment of ecological predictors. 2017. Bird Study. DOI: 10.1080/00063657.2016.1249821