Aspects of the biology of the shag (Phalacrocorax aristotelis)

On the Isle of May, Scotland, large numbers of Shags have been marked annually since 1962. From 1981 to 1983, this marked population was sampled to investigate retrospectively a catastrophic decline in numbers of breeding Shags between 1974 and 1976. An electrolytic method was developed to read incomplete ring-numbers on abraded rings: it was 94% successful. The adult annual survival rate before, during and after the decline remained constant at 87%; during the decline, up to 60% of experienced adults refrained from breeding, laying was a month later than usual, chick production and post-fledging survival were both abnormally low. Failure of the fish stocks around the Isle of May probably caused the decline. Dispersal, pair-bond and reproductive performance with respect to age, timing of breeding and nest-site quality were also examined. Natal and breeding fidelity were strong, and more pronounced in males. Second-year males, breeding for the first time, performed half as well as older males; the effects of other male age categories and female age were unimportant. A strong age-independent seasonal decline in breeding performance was attributable to both environmental factors and individual quality. Four nest-site quality criteria affected reproduction: ledge size, dampness, exposure, and vulnerability to the sea. Experienced Shags bred early and occupied good sites; the social structure forced later-breeding recruits onto poorer sites within the breeding group. Shags which changed sites between years preferred those with a previous history of occupation. Sites occupied continuously were of highest quality. The study population currently shows no sign of density-dependent regulation; potential future regulatory factors are food and a shortage of good quality nest-sites.