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.

Convergent evolution is a central theme in biology. Birds are an ideal system to examine the mechanisms underlying convergent evolution. Although bird patterning is diverse, within-feather patterns have repeatedly converged on the same four types: mottled patterns, scales, bars and spots. Other avian patterns occur, e.g. stripes, but are rare. In my thesis I examine the four main mechanisms underlying convergent evolution in plumage patterns: evolutionary genetics, evolutionary development, natural selection for signaling and camouflage. Japanese quail (Coturnix japonica) is a model system in developmental biology. Examining the developmental basis of pattern formation using molecular techniques, the dorsal patterning of embryonic quail is likely due to activation of the melanocortin-1 receptor, which is a highly conserved pathway in vertebrates. I examined whether a reaction-diffusion based theoretical model of pattern formation may predict developmental constraint in two groups that have different lifestyles and spectacular patterns: waterfowl (Anseriformes) and gamebirds (Galliformes). Tracing the evolutionary trajectory of pattern evolution with Bayesian comparative modeling there was evidence for developmental constraint in pattern evolution. Adaptive explanations may also result in convergence. Cuckoo-hawk mimicry has been demonstrated in the common cuckoo (Cuculus canorus) and the Eurasian sparrowhawk (Accipiter nisus), but may be prevalent in Old World cuckoos. Randomly selecting a parasitic cuckoo from each genera of Old World cuckoos and <8 sympatric raptors, I quantified their barred patterns using digital image analysis and found that parasitism can explain convergent evolution in the patterns of parasitic cuckoos and raptors. Patterns may have evolved due to ecological selection. Examining the patterns of 80% of all avian species worldwide, I found that habitat does not predict patterning, and that all four patterns are found in all habitats. These results demonstrate that the mechanisms of convergent evolution are diverse, and that development and natural selection have contributed to pattern evolution.