The eggshell colouration among and within certain bird species is highly variable. Although many studies have addressed this variation, the reasons for it remain largely unclear. Consistent individual differences in egg colouration may improve the ability of colonially breeding birds to recognise their clutch among other neighbouring nests. Moreover, in species with a high incidence of intraspecific brood parasitism, such as the Black-head Gull Chroicocephalus ridibundus, egg colouration patterns may allow parents to expel foreign eggs from their own clutch. Here, we used standardised photography and image processing, including UV spectral information, to investigate egg colour patterns in female Black-headed Gulls (476 eggs in 170 clutches) during four breeding seasons, from 2015 to 2018 in NE Germany. We confirmed maternity by comparing DNA extracted from eggshells with feathers from the breeding female. Our results showed a greater similarity of eggs within a clutch than among clutches (ANOSIM: R = 0.43, P = 0.001). Black-headed Gulls showed consistent individual-specific eggshell colour pattern across consecutive breeding seasons. Genetic analyses further revealed one foreign egg was present in three out of 25 clutches (12%), but these eggs were not expelled by the gulls, possibly because these eggs did not differ significantly in colouration from their own. Additionally, we conducted a clutch exchange experiment to assess the reaction of the breeding pair to a foreign clutch. After 21 neighbouring clutches had been exchanged, adults came back immediately and incubated the interchanged clutch without any negative reaction. It seems that they do not use the specific colour pattern for recognition of their clutches, but possibly use other cues that remain to be investigated. While the function of individual egg colour patterns remains unclear, our results strongly suggest colour variation is driven by internal rather than external factors in this colonially breeding species.
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Vol. 108 • No. 1