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Breeding and population ecology of sooty terns on Ascension Island

Breeding and population ecology of sooty terns on Ascension Island
B. John Hughes


School of Biosciences, University of Birmingham, Birmingham B15 2TT, UNITED KINGDOM.


Seabird breeding success and population size on many oceanic islands have declined in recent years as a result of predation by non-native mammals. On Ascension Island, South Atlantic the sooty tern Onychoprion fuscatus breeding population declined precipitously in size in the 1970s. Despite cat eradication in 2003 and a cull of predatory mynas, the tern population between 1990 and 2012 consistently contained 350,000 birds. Shortages of small fish within foraging range of Ascension driven to the sea surface by once more prevalent tuna rather than cat predation probably caused the 1970s decline. Abundance and size of rats increased in response to cat eradication with the impact of rats on the size of the seabird population similar to the combined impact of both cats and rats. Ascension sooty terns have adopted a life-history strategy of sub-annual breeding as a viable alternative to seasonal breeding. The population consistently bred every 289 days, as did most individual birds. Despite an increase in lifetime reproductive rate resulting from sub-annual breeding, I predict a slow but sure decline in the population size unless rats are controlled. Studies of sooty tern migratory ecology are required and the species’ current IUCN Red List category should be reviewed.