Predation pressures on sooty terns by cats, rats and common mynas on Ascension Island in the South Atlantic

Bernard Hughes, Roger Dickey, Silas Reynolds

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Despite the presence of invasive black rats (Rattus rattus), common mynas (Acridotheres tristis), and feral domestic cats (Felis catus), sooty terns (Onychoprion fuscatus) breed in large numbers on Ascension Island in the tropical South Atlantic Ocean. These introduced predators impact the terns by destroying eggs or interrupting incubation (mynas), eating eggs (mynas and rats), eating chicks (rats and cats), or eating adults (cats). Between 1990 and 2015, 26 censuses of sooty terns and fi ve of mynas were completed and myna predation was monitored on 10 occasions. Rat relative abundance indices were determined through trapping around the tern colonies and rat predation monitored by counting chick carcasses. Cat predation was quantifi ed by recording freshly killed terns. Prior to their eradication in 2003, cats had the greatest impact on sooty terns and were depredating 5,800 adults and 3,600 near-fl edging chicks (equivalent to the loss of 71,000 eggs) each breeding season. We estimated that 26,000 sooty tern eggs (13% of all those laid) were depredated by approximately 1,000 mynas. Rats were not known to depredate sooty terns prior to cat eradication but in 2005, 131 of 596 ringed (monitored) chicks (22%) were depredated by rats. In 2009 chick carcass density was 0.16 per m2. Predation by rats hugely increased in the absence of cats and was the equivalent of 69,000 eggs. Care is needed when applying our findings to seabirds globally. The scarcity of alternative food sources and seasonally high density of easily available prey in the sooty tern colony may have magnified predation by cats, rats and mynas.
Original languageEnglish
Title of host publicationIsland Invasives: Scaling up to Meet the Challenge
Subtitle of host publicationOccasional Paper of the IUCN Species Survival Commission No. 62
EditorsC. Veitch, M. Clout, A. Martin, J. Russell, C. West
Place of PublicationGland, Switzerland
PublisherInternational Union for Conservation of Nature and Natural Resources
ISBN (Electronic)9782831719610
ISBN (Print)9782831719627
Publication statusPublished - 5 Apr 2019

Publication series

NameEdit Occasional Paper of the IUCN Species Survival Commission
PublisherInternational Union for the Conservation of Nature and Natural Resources


  • Non-native species
  • population size
  • predation rate
  • sooty tern
  • United Kingdom Overseas Territory (UKOT)


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