Summary (AGH): Seabirds, such as gulls and terns, nest on land, but find most of their food by ranging over often vast areas of the ocean. By studying their diets, and how they change over time, we can get indications of marine ecology and its changes over time that would require enormous effort to get using other methods. Herring and Great Black-backed Gulls regurgitate the undigestible parts of their food in pellets, like the better-known pellets that owls leave behind, giving a window into what they eat. Pellet contents collected in the 1970s had more offshore prey and terns (including tern eggs), and fewer mollusc, crab, and seal tissue than those collected in 2011-2012. Also, the more recently collected pellets showed that, despite much variation with time of year across individual gulls, the two gull species showed consistent differences in diet. Specifically, the chemical composition of the pellets, measured by chemicals that differ in how radioactive they are, showed that Great Black-backed Gulls ate food that was higher in the food chain than Herring Gulls. The changes in diet for both gulls over time might be related to declines in fish abundance, greater difficulty in preying on terns (because their populations have increased, and thus gain protection in numbers), and a huge increase in the island’s seal population. At the same time, the differences in diet between the two species suggests these two species might co-exist on the island partly because they feed on different prey.
