The Azores Archipelago holds the second most important population of Roseate Terns in the North Atlantic. However, the size of the population has been decreasing. In this study, we used remote-sensing cameras and visits to the main colonies of Common Terns Sterna hirundo and Roseate Terns S. dougallii on Terceira Island to identify the causes of breeding failure. Nest depredation by introduced mammals, particularly rats and cats, was the main cause of breeding failure, leading, in some cases, to the complete destruction of the colony.
Additional threats included nest disturbance by humans and dogs, which caused the destruction of some nests. Measures aimed at mitigating depredation were tested but in the case of rodents, control plans proved difficult to achieve. This is probably because rodents are widespread in the Azores, they are not limited by food resources, and/or trapping methods are ineffective.Control actions were conducted under approval from the Azores Government, Regional Environmental Directorate. Partial funding came from both the Portuguese Foundation for Science and Technology (FCT) and the Portuguese Ministério da Educação e Ciência. Specifically, the strategic project UID/MAR/04292/2013 was granted to the Marine & Environmental Sciences Centre, and postdoctoral grants were awarded to VCN by the Regional Fund for Science and Technology through grant FRCT/M3.1.a/F/072/2016. LLL was supported by a grant from the Fundação para a Ciência e Tecnologia-FCT (SFRH/BD/115022/2016). We are grateful to Kirstin Jones for comments and suggestions that improved an earlier version of the manuscript, along with English proofreading. We thank Patricia Pedro, Maria Magalhães, Matt Cormons, Cristina Nava, Lune Fleur, and the nature wardens of the Terceira Natural Park for help with the fieldwork. We are also very grateful to the Director of Terceira Natural Park and the tern colonies' management team, as well as Rita & Alexandre from Octopus for the availability of boat transport to the islet and the reviewers whose efforts improved our paper.info:eu-repo/semantics/publishedVersio
