Annual monitoring of breeding seabirds on the Isle of Tiree 2004-13 revealed significant declines in the numbers of Northern Fulmar Fulmarus glacialis, Common Guillemot Uria aalge, Razorbill Alca torda and Black-legged Kittiwake Rissa tridactyla at the main seabird colony of Ceann a' Mhara. Numbers of Herring Gull Larus argentatus and Lesser Black-backed Gull L. fuscus also declined significantly across the island over the same period, whilst numbers of Common Gull L. canus increased significantly. Most other breeding seabirds including European Shag Phalacrocorax aristotelis, Black Guillemot Cepphus grylle, Little Tern Sternula albifrons, Arctic Tern Sterna paradisaea and Black-headed Gull Croicocephalus ridibundus remained stable or showed non-significant declines. Breeding was confirmed for the first time on Tiree by single pairs of Great Skua Stercorarius skua and Atlantic Puffin Fratercula arctica. Annual estimates of breeding success were recorded for six species. There were complete breeding failures of Black-legged Kittiwake in 2007-08, Arctic Tern in 2004-05 and Little Tern in 2006, although all three species had higher breeding success in 2009-13.

The island of Tiree (56&°;30'N 6&°;54'W) in Argyll and Bute, the westernmost of the Inner Hebrides, is flat, low lying and roughly 20 km long and 5 km wide, and has long been known for its important bird populations (Bowler & Hunter 2007). Coastal habitats include small rocky islets, extensive sandy bays, low rocky shores and higher cliffs on the western and southern coasts. Shell-sand rich machair fringes the coast, with areas of in-bye grassland wedged between this and the low grazed wet heathland in the centre of the island, all interspersed by extensive areas of freshwater marsh, pools and larger lochs. The island remains free of North American Mink Mustela vison, ferrets M. putorius furo and Red Foxes Vulpes vulpes and despite the presence of a recently introduced population of European Hedgehogs Erinaceus europaeus and a large population of Brown Rats Rattus norvegicus, it supports a varied assemblage of breeding seabirds: over 6,000 pairs of 14 species in 1999 in the Seabird 2000 census which are important in a regional context (Mitchell et al. 2004; ap Rheinallt et al. 2007). All of the Black-legged Kittiwakes Rissa tridactyla, Common Guillemots Uria aalge, Razorbills Alca torda and European Shags Phalacrocorax aristotelis, and the great majority of the island's Northern Fulmars Fulmarus glacialis and Black Guillemots Cepphus grylle, nest on
60 m high cliffs of the headland at Ceann a' Mhara, whilst the gulls and terns nest at scattered inland and coastal colonies around the island (Broad & Cadbury 1989). Seabird breeding success was believed to have been generally good on the island between 1985 and 2001, but mass breeding failures were first noted amongst Arctic Terns Sterna paradisaea and Black-legged Kittiwakes in 2002-03. An unprecedented influx to the island's lochs of some 3,000 adults of the latter species on 25 June 2002, presumably also involved failed breeders from elsewhere (Bowler & Hunter 2007). These events stimulated more systematic annual monitoring thereafter.

I am greatly indebted to all the coordinators and fieldworkers of the earlier surveys on Tiree not named elsewhere, particularly Roger Broad and James Cadbury in
1986–88 and Alan Leitch in 1999–2000 and to RSPB Scotland and Scottish Natural Heritage for supporting the work financially. Ben Jones (RSPB) and David Jardine kindly supplied data from Coll and Colonsay respectively, whilst Roddy Mavor kindly sent me SWT data from Eigg. Mark Bolton and Jeremy Wilson (both RSPB) made helpful comments on an earlier draft. Simon Foster, Amanda Kuepfer, Roddy Mavor and Martin Heubeck all made very detailed and pertinent comments, which strengthened the paper considerably.

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