Causes of mortality of Black-fronted Terns Sterna albostriata on the Ohau River, South Island, New Zealand

Abstract

We examined the carcasses of 148 Black-fronted Terns Sterna albostriata found during the 1998-2000 breeding seasons on the Ohau River, South Island, New Zealand. Predation was the primary cause of mortality of adults, juveniles and chicks, resulting in 47% of all deaths. Video footage showed Feral Cats Felis catus were responsible for 6% of all predator-caused deaths, and physical evidence at carcass remains linked cats to another 19% of predations. Further evidence suggested Norway Rats Rattus norvegicus and Stoats Mustela erminea were responsible for 51% and 6% of predator-caused deaths, respectively. The prey remains left by the different predator species are described. Other causes of mortality included starvation (4%), power lines (1%), road traffic (1%) and various natural causes of mortality (5%). Causes of mortality could not be assigned to 41 % of Black-fronted Terns, but most of these were young chicks that died at the nest soon after hatching. We suggest that predators are potentially the main cause of population decline in black-fronted terns and that predator control targeting cats and Norway Rats during the breeding season should be trialled.