Decline in black-legged kittiwakes Rissa tridactyla on Scilly: Investigating the possible role of predation by large gulls (Larus spp.)

1. Across the U.K the population of black-legged kittiwakes has declined by >50% since 1990 alone. The total number of kittiwake pairs breeding on the Isles of Scilly during 2006 stands at 266 compared to around 1,400 in 1970. Recent monitoring of the islands largest sub-colony the island of Gugh indicates almost complete reproductive failure over the past four breeding seasons. 2. Three large gull species breeding on Gugh herring Larus argentatus, lesser black-backed Larus fuscus, and greater black-backed gulls Larus marinus are known to predate on other seabirds inhabiting the islands. The aim of this study was to investigate possible causes for the recent decline in the kittiwake population on the largest sub-colony Gugh, with particular attention to the possibility of predation by these gulls through study of their diet. . This was used with observations of the Gugh kittiwake colony to discover any evidence of kittiwake/gull interactions whilst recording nest attendance of adult kittiwakes. 3. All 3 gull species showed differences in diet composition throughout the study as was expected due to the dietary generalisation exhibited by these gull species, dependent on changes in available prey source. However there was an absence of kittiwake adults and chicks in gull diet, and few kittiwake/gull interactions. 4. Repeated predation attempts by carrion crows on kittiwake eggs were observed on Gugh kittiwake nests and the colony failed to fledge chicks for the 5th year in a row. Carrion crows were not observed predating on kittiwakes on St. Martins or a lone pair on St. Agnes, Isles of Scilly, and they fledged 0.56 and two chicks respectively. 5. Given the current severity of the decline in population of kittiwakes on Gugh and only 2 or 3 carrion crow are involved in kittiwake egg predation at one location within the islands. Removal of these individuals, although controversial may be the best option to allow this important colony to recover, along with regular monitoring of the Isles of Scilly kittiwakes throughout their egg laying period to monitor any more interactions with crows at other colonies.Faculty of Science & Technolog