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Durham e-Theses
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Aspects of the biology of the shag: (phalacrocorax aristotelis)

Aebiacher, Nicholas J. (1985) Aspects of the biology of the shag: (phalacrocorax aristotelis). Doctoral thesis, Durham University.

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Abstract

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.

Item Type:Thesis (Doctoral)
Award:Doctor of Philosophy
Thesis Date:1985
Copyright:Copyright of this thesis is held by the author
Deposited On:18 Sep 2013 09:25

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