Causes and consequences of variations in eggshells in the lesser black-backed gull
Variations in egg size and egg composition between females of the same species, as well as among eggs in the same clutch, have been studied in many avian species. The eggshell serves crucial functions in avian reproduction such as protection of the embryo from mechanical damage and from the invasion of micro-organisms, source of calcium to the embryo, control of gas exchange with the environment and conservation of water. But little attention has been paid to variation in eggshell, especially within-clutch variation. This thesis focuses on variations in eggshell characteristics in relation to laying order in a single species, the lesser black-backed gull (Larus fuscus). In order to evaluate a proper method for measurement of eggshell characteristics, this thesis used more than one technique to measure shell thickness, shell porosity and shell coloration. For the measurement of shell porosity, two techniques for counting pores were validated for the first time in this thesis. This study found within-clutch variations in shell porosity, mammillary layer contact area and shell coloration but not in shell thickness. The last-laid egg had a larger mammillary layer contact area and often had paler shell colour and streaks on the shell. This study found some relationships between shell structures and shell coloration. A calcium-supplementation experiment was used to investigate whether the shell formation is limited by calcium-availability. This thesis found effect of calcium-limitation on shell thickness, but no effect on shell background colour.