Integrated seabird monitoring studies on the Isle of Canna, Scotland 1969-99
Swann R.L. (2000) Atlantic Seabirds 2(3/4):151-164
An annual survey of breeding seabird numbers on the Isle of Canna, western Scotland, was established in 1969; the breeding productivity of several species is also monitored each year. Over 53,000 seabirds have been ringed since the study started and the subsequent recoveries and recaptures have been used to assess changes in survival rates and return rates to the island. Northern Fulmars Fulmarus glacialis have shown great fluctuations in numbers since 1973 and from 1996-99 there has been a noticeable decline accompanied by a decrease in breeding output. The percentage of study burrows occupied by Manx Shearwaters Puffinus puffinus began to decline in the late 1980s and breeding success has also declined; the species is now close to extinction on the island. Shags Phalacrocorax aristotelis, Kittiwakes Rissa tridactyla and Common Guillemots Uria aalge all showed a steady increase in numbers up to the mid 1980s, after which numbers declined or, in the case of Guillemots, stabilised until the early 1990s when further increases took place. During the periods of population growth return rates of young Shags and Guillemots were high but they subsequently declined during the period of population decline/stability and in the case of Guillemot this was linked to a significant increase in first-year recovery rates. The period of decline was also associated with a drop in Shag and Kittiwake breeding output. These changes were probably driven by fluctuations in the food supply, although increased predation might have affected Fulmar and Manx Shearwater numbers.
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