Using radio-transmitters to improve estimates of gull predation on Common Eider ducklings

Abstract

We quantified the efficacy of using pellet numbers to estimate predation rates on Common Eider (Somateria mollissima) ducklings by Great Black-backed Gulls (Larus marinus) by using eider ducklings fitted with radio transmitters. Ducklings fitted with radiotransmitters were no more vulnerable to gull predation than were other ducklings. The recovery of radio-transmitters attached to eider ducklings and subsequently retrieved from Great Black-backed Gull pellets suggests that traditional methods of estimating the number of eider ducklings eaten by gulls from the remains found in pellets at gull nests and loafing areas underestimates the true number eaten by gulls by a factor of 5-17. Previous low estimates of eider duckling mortality on the Wolves Archipelago, Bay of Fundy cannot be explained by movements of broods to the mainland coast.