Retrospective Investigation of an Ecological Disaster in the Shag, Phalacrocorax aristotelis: A General Method Based on Long-Term Marking

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Abstract
(1) Between 1974 and 1976, the numbers of shags breeding on the Isle of May, S.E. Scotland, crashed from 1076 to 497 pairs. Before 1974, the colony had been increasing steadily at an average annual rate of 15.6%. (2) Sustained ringing of both adults and young since 1962 made possible a retrospective investigation, in 1982, of the population dynamics of the colony. (3) The adult annual mortality rate remained unchanged over the critical period; it was apparently greater in old birds. Emigration was low (nil in 1974-76); immigration was negligible. The crash was due to extensive non-breeding by experienced adult shags: simulation showed that the proportion of adult non-breeders was 0-30% in 1974, and 25-60% in both 1975 and 1976. Pesticides, heavy metal poisoning and paralytic shellfish poisoning were not implicated. (4) Egg-laying was abnormally late in 1974-76 and annual young production dropped from 1.16 to 0.77 fledglings per pair. The first-year survival rate was 56% in normal years and 17% in crash years, when the peak in first-year mortality shifted from late winter to the autumn. (5) Climate and the abundance of food accounted independently for 68% of the variation in annual laying date. March was colder than average in 1974-76, herring and sandeel stocks were unusually low. Failure of the food supply was probably the main reason for poor breeding in those years.