The Age-Specific Breeding Performance of Great Skuas on Shetland

Ratcliffe, Norman (1993) The Age-Specific Breeding Performance of Great Skuas on Shetland. PhD thesis, University of Glasgow.

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The effect of age on the breeding performance of the great skua was examined between 1991 and 1993. There was a linear increase in clutch volume, aggression towards human intruders and fledging success with age while laying date became earlier with age. Clutch size and hatching success improved with age in a non-linear manner; increasing in younger birds before reaching a plateau in older birds. There was no effect of age on inter-nest distance nor on the condition of either the 'A' or 'B' chick. Female age was important in the advancement of laying date while male age was more important in increasing clutch volume. The latter result was possibly due to older males having a higher rate of courtship feeding. An improvement in food supply between 1989 and 1993 reduced the number of birds deferring breeding on territory, advanced laying date, increased clutch volume and greatly improved fledging success. The increase in clutch volume with improved food supply was statistically independent of laying date. Food supply had an additive effect on age specific laying dates but there was no significant interaction. There was a significant interaction of food supply and age on fledging success; in years of intermediate food supply fledging success improved with age, while in good years the fledging success of younger birds was elevated to a level similar to that found in older ones. This suggests that the fledging success of young birds was constrained by food supply to a greater extent than in older birds. Breeding experience had an effect on laying date, clutch size and clutch volume but did not affect hatching success. Age and experience were closely inter-related but separating the effects of these factors on breeding performance was not possible due to small samples of birds in which both age and experience were known. Despite this it seems that clutch size improved due to breeding experience rather than age. Improvements in laying date, clutch size and clutch volume were found in individual birds on successive breeding attempts following recruitment suggesting that individual birds improve breeding performance through a maturation process or a learning of skills beneficial to breeding. There was no evidence for an increase in reproductive effort with age, despite the fact that a decrease in survival was found with old age, which would provide a selective basis for such an increase in effort. There was also evidence to suggest that high fledgling production produced a cost of reproduction in terms of a decline in survival, this only being evident in old birds. Low clutch size and aggression was associated with increased mortality, this finding providing evidence that poor quality breeders have lower life expectation. The high mortality of poor quality birds which laid only one egg was especially evident in first time breeders. This effect could produce and increase in clutch size with age since birds laying a one egg clutch will die early while birds which lay 2 eggs will be represented in the population at older ages.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Bob Furness
Keywords: Zoology, Ecology
Date of Award: 1993
Depositing User: Enlighten Team
Unique ID: glathesis:1993-76293
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 16:09
Last Modified: 19 Nov 2019 16:09

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