Hormonally mediated maternal effects in birds

The main aim of this thesis was to investigate the effects of environmental conditions, particularly unpredictable or potentially negative ones, on the maternal transmission of the primary avian stress hormone, corticosterone, to developing embryos. We currently lack information on the extent to which conditions in the maternal environment are transmitted to the offspring in birds via egg compositional changes. It is possible that maternally derived hormonal signals communicate information about the external environment to developing embryos and directly influence the fitness of their offspring in a negative or positive way. I found, using captive zebra finches, that the experimental stressor of unpredictable food availability (as these birds are used to ad libitum food) experienced by mothers can elevate yolk CORT concentrations, but only when combined with the additional demand of laying a replacement clutch (potentially a buffering system to prevent mild stressors impacting on CORT transmission to the embryo). I then looked at yolk CORT concentrations in two populations of gulls (herring and lesser black-backed gulls) in which the population trajectories differed depending on environmental conditions (potentially a reflection of different exposures to stressful stimuli). The results however did not support this hypothesis, as there were no differences according to habitat type or between species (where they coexist). This would suggest that the different environmental circumstances (harsher for the herring gull) experienced by these two species are not reflected in differences in their eggs (at least in terms of CORT). This could be the result of the eggs being buffered from the maternal CORT environment or it may be that the difficult environmental conditions are not occurring during the breeding season. We also identified that experimental human disturbance during the laying period does not appear to elevate yolk CORT concentrations, although there was a trend for concentrations to be higher following the loss of the first clutch in the herring gull (as seen in the zebra finches). I also measured yolk CORT concentrations in Common Eider eggs and looked for differences according to the degree of nest shelter. I found no relationship between shelter and yolk CORT, but birds that laid in more sheltered sites had, on average, smaller eggs. This may indicate lesser quality birds are nesting in the sheltered sites and that yolk CORT is not affected by maternal condition. Finally, I looked at another mechanism through which information relating to the maternal environment could be transferred to the embryo. I investigated whether there were any links between maternally derived immunity and CORT by comparing the anti-microbial lysozyme and CORT concentrations in the albumen. I found no correlation between CORT and lysozyme, suggesting that CORT may not affect lysozyme production. It may be that other factors such as colony density and ‘cleanliness’ are more important in determining the concentrations of lysozyme deposited in the egg or that lysozyme production is not sufficiently costly to be influenced by the maternal stress state. The overall theme of my findings is that CORT concentrations in eggs do not appear to vary much with maternal environments. I will discuss these findings in their broader ecological and evolutionary context and discuss if stress hormones are indeed being used as adaptive signals for preparing the embryo for its postnatal environment