Effects of predation pressure and nest-site selection on the breeding success of terns in a Finnish archipelago

Studying and quantifying the breeding success of species can help to understand population trends and provide conservation guidance. Here, we studied the breeding success of Arctic Tern (Sterna paradisaea) and Common Tern (Sterna hirundo) colonies in a Natura 2000 site of the Western archipelago of Finland aiming to understand which factors related to nest-site selection and predation pressure best explain breeding success. We monitored 72 tern nests at 4 colonies and observed them during standardized monitoring sessions to record predation attacks on the tern nests. We ran generalized linear models to determine the factors which impact breeding success (hatching and fledging success). Hatching success was high across the 4 colonies with 69% of the eggs hatching, whereas fledging success was low with only 1 colony producing 14 fledglings while all other colonies had zero fledglings surviving (12% total fledging success). Regarding nest-site selection, our results demonstrate that a greater proportion of vegetation cover increases breeding success, likely through better nest concealment against predators. Nests in smaller colonies with a higher nest density and located closer to the colony center also produced more hatchlings and fledglings. Most predation likely happened during the chick stage, while our extrapolation predicted that predation accounts for at least 94% of all chick losses, indicating that predator control would aid breeding success of tern colonies in West Finland substantially.