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Hughes, B.J.; Martin, G.R.; Reynolds, S.J.
Cats and seabirds: effects of feral domestic cat _Felis silvestris catus_ eradication on the population of sooty terns _Onychoprion fuscata_ on Ascension Island, South Atlantic
2008  Ibis (150): 122-131

The population of Sooty Terns _Onychoprion fuscata _breeding on Ascension Island in the Atlantic Ocean was monitored over 17 years (1990-2007). This period spanned the programme of feral Domestic Cat _Felis silvestris catus _eradication from the island, which commenced in 2001 with the last Cat recorded in 2004. We report on the abundance of Sooty Terns and Black Rats _Rattus rattus _before and after Cat eradication. The Sooty Tern breeding population in the 1990s averaged 368 000 and Cats were killing Terns at an average rate of 33 adults per night. Following Cat eradication, adult Terns are no longer predated. However, egg predation by both Rats and Common Mynas _Acridotheres tristis_has continued with Mynas destroying more eggs than Rats. Unexpectedly, we observed a change in Rat predatory behaviour. Following Cat eradication, Rats have become a major predator of Sooty Tern chicks. Despite this change, the Tern population has shown a season-on-season increase since Cat eradication, 48.8% in 2005, 8.2% in 2006 and 6.1% in 2007, and the breeding population increased to 420 000 birds in 2007. Incubation success improved from66.0 to 84.4% during Cat eradication, before dropping down again to 67.9% after Cats were eradicated and Rat control measures were introduced. Index trap lines were set for Rats and Rat numbers fluctuated widely immediately after Cats were eradicated but there were no significant differences that could be attributed to changes in Cat numbers. Ascension Island Sooty Terns breed every 9.6 months and juveniles defer breeding for seven seasons. Hence2008 is the first year in which an increase in the breeding Sooty Tern population directly attributable to Cat eradication is likely to be detected. We conclude that long-term monitoring is essential to guide conservation practice even in this relatively simple predator-prey system.

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