Diet Composition and Reproductive Performance in Central Europe's Last Gull-Billed Tern Population – a Long-Term Study

As more and more species are in decline, understanding the various aspects underlying their population dynamics is crucial to optimise conservation efforts. Hereto, we studied the diet composition and reproductive performance of Gull-billed Terns forming the last breeding population in Central Europe by analysing population-level data collected between 2011 and 2022 as part of a conservation program. We show that the main prey types vary among breeding stages and across years. When observed foraging for themselves, Gull-billed Terns were mainly found to eat insects, whereas crabs and worms were the main prey items used for courtship feeding and chick provisioning. The proportion of prey of terrestrial origin used for courtship feeding increased over the study period, whereas no such change was observed in prey used for self-feeding or chick provisioning. Clutch and egg size also increased over the study period but were not correlated with the proportion of terrestrial prey used in courtship. Fledging success did not change over time and was not correlated with variation in chick diet. These results confirm that the Gull-billed Terns in this population are generalist feeders and suggest that their breeding success is not currently threatened by a lack of, or changes in, specific prey types. The shift towards more terrestrial prey in part of their diet may, however, indicate changes in the aquatic ecosystem, which should be further assessed, together with other potential risks for these threatened birds, such as inbreeding and pollution.