Egg Production in Lesser Black-Backed Gulls: An Experimental Approach

Bolton, Mark Richard (1991) Egg Production in Lesser Black-Backed Gulls: An Experimental Approach. PhD thesis, University of Glasgow.

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1. A new technique to estimate pectoral muscle protein condition from body measurements which can be taken from live lesser black-backed gulls (Larus fuscus) is described. The method uses the profile of the pectoral muscles over the keel to estimate the maximal muscle cross sectional area. The product of cross sectional area and muscle length provides an index of muscle volume, which, together with body weight provides an accurate predictor of actual pectoral muscle protein (muscle lean dry weight) determined by carcass analysis, in a stepwise regression procedure. The regression model was validated using a second, independent sample. Different methods of controlling for body size to calculate protein condition from measures of total protein were considered. 2. The consequences of variation in egg size for offspring survival to fledging in lesser black-backed gulls, was examined using a clutch transfer experiment. The experiment tests the hypothesis that parents producing large eggs also provide better parental care for their offspring than those which lay small eggs, which explains part of the correlation between egg size and offspring survival in non-manipulative studies. Clutches of large eggs were cross fostered with clutches of small eggs in order to examine the egg size and parental effects independently. Large eggs produced chicks which were not only skeletally larger (tarsus length) but also heavier for their size than those from smaller eggs, both of which could contribute to increased chances of survival. However, logistic regression analysis showed that the parental effect was more important in determining chick survival to fledging, though egg size does have a separate, independent, but smaller effect. This appears to be mediated by differences in chick skeletal size rather than condition. Since large egg size is advantageous, selection for increasing egg size is implied. The likely counterbalancing forces are discussed. 3. Experimental evidence is provided to demonstrate that a reduction in clutch size and egg size among unfed lesser black-backed gulls in one year was the result of food limitation of egg production. The average clutch size of unfed birds was among the smallest recorded for the species, and significantly lower than in previous years at the same colony. Pairs provided with supplementary food in the same year produced significantly larger clutches and larger eggs than controls, which were typical for the species. Provision of additional food at a second colony, in a different year, where egg production of unfed birds was normal, had no effect on either clutch size or egg size, suggesting that food limitation of egg production is not the cause of the normal limit to clutch size at three eggs. 4. The nutritional requirements of egg formation were examined through a series of pairwise feeding experiments. Provision of either fish or fat supplements to different experimental groups in the year of depressed egg production showed that the reduced clutch and egg sizes of unfed birds were the result of limited protein, not energy intake. Supplements of two different protein types (fish and egg) in a further year, when egg production of controls was good, demonstrated that egg production (in this case egg size only) could be limited by availability of specific nutrients, possibly certain essential amino acids, provided by the egg but not the fish protein supplements. 5. The role of female body condition in providing a causal link between food supply and egg production was investigated using the pectoral profile method outlined above. There was a significant correlation between pectoral muscle protein condition at the start of laying and the number of eggs laid among females. There was no relationship between body condition and either clutch size or egg size among males. In addition, a treatment-based analysis showed that in the year of food limitation, unfed and fat-fed females, which laid small clutches, were in poorer condition at the start of laying, than those receiving the protein supplement, which laid larger clutches. In the following year, when there were no differences in clutch size according to prelaying feeding regime (though egg-fed birds laid eggs 10% larger than controls), there were no differences in body condition at the start of laying between treatments. These results suggest a causal role of flight muscle protein condition in the determination of clutch size but not egg size. 6. The current selection pressures involved in the usual truncation of the clutch at three eggs were investigated using a series of clutch size manipulations. The costs of egg laying and egg rearing were independently manipulated to determine whether the limit to clutch size is currently influenced by egg production costs, incubation capacity or chick rearing ability. Females which laid a clutch of further three eggs following the removal of their first egg tended to rear fewer chicks to fledging than those laying unmanipulated clutches of three eggs, but the differences did not quite reach significance. Incubation efficiency was not reduced in four egg clutches which were produced by adding an extra egg to recently completed clutches, and there was no indication of reduced hatchability among last-laid eggs of enlarged clutches, compared with controls. Broods of four were not more productive than control broods of three. The upper limit to clutch size may be determined by brood rearing capacity, which could act in conjunction with the suggested effects of increased egg production. Four chick broods did not produce more fledged young than control broods of three due to increased chick mortality early in life. It is suggested that the increase in hatching asynchrony which inevitably accompanies the production of a four chick brood may be the cause of the increased mortality, and may set the upper limit to clutch size at three eggs.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Ecology
Date of Award: 1991
Depositing User: Enlighten Team
Unique ID: glathesis:1991-78336
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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