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Title: Aspects of population dynamics and breeding ecology in the gannet, Sula bassana (L.) on Ailsa Craig
Author: Wanless, Sarah
ISNI:       0000 0001 1629 0215
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1979
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This study was undertaken to investigate reports of periodic large fluctuations in the Gannet Sula bassana (L) population on Ailsa Craig between 1936 and 1976. Also, aspects of the breeding ecology and behaviour of a west coast gannetry (Ailsa) were to be compared with data collected by Nelson (1965, 1966c) from the Bass Rock on the east coast. Regular photographic counts were made between September 1974 and April 1977. The number of occupied sites was counted directly from the photographs. Sample counts of breeding pairs and non-breeding nest or site holders were made from the ltind, and the ratio used to correct each, section to give separate totals of breeders and non-breeders. Counts are thought to be accurate to within 7 per cent of the mean. In 1975 and 1976 colony occupation followed an approximately normal distribution, with a gradual spring build-up, a summer peak and a decline during late September and early October. Breeding pairs made up about 80 per cent of the mean mid-April/early May totals. During May and early June there was a 20 per cent increase in the number of occupied sites, so that proportions of sites held by breeders and non-breeders during the summer were about 60 per cent and 40 per cent respectively. A few birds were present in the club areas in April. Numbers remained low until June when there was a rapid increase. Maximum numbers were 245 birds in 1975 and 707 in 1976. Numbers declined during August and early September. Although the seasonal patterns of occupation were similar in 1975 and 1976 counts in the latter were consistently higher, particularly early in the season. The breeding population increased by 6 per cent from 9670 pairs to 10200 pairs, and the maximum number of occupied sites increased from 14880 to 16220, an 11 per cent increase. The latter was largely made up of non-breeding site holders. Between June and September the number of occupied sites showed considerable variation (up to 17 per cent over a 10 day period). In 1975 and 1976 the difference between the mid-April count and the summer maximum was about 30 per cent. Thus isolated counts made at different times of the season could lead to the conclusion that the population shows massive annual fluctuations. The number of chicks fledging in 1975 and 1976 were estimated at 6740 and 7110 respectively. Comparison of the totals obtained in this study using photographs and Gibson's comparable direct counts showed good agreement when the former were corrected to give occupied nests. Fluctuations in annual counts can largely be explained in terms of counting error and seasonal variations in the number of non-breeding birds present in the colony. Differences in population dynamics of west and east coast gannetries are discussed in detail, with particular reference to colony interchange amongst the former.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available