Looking at both sides now: Examining trade-offs in ecological immunology in birds

The immune system plays a crucial role in animal survival and reproduction, but excessive investment in immune defences can bring energetic costs and self-damage, resulting in trade-offs with other vital activities such as migration, reproduction, and sexual selection. Ecological immunology has emerged to understand how immune defence is shaped by ecological and life-history factors, including environmental conditions. In this thesis, I use birds as model organisms and apply a multi-dimensional approach that combines phylogenetic comparisons, common garden experiments, field observations, and viral infection studies to explore variation in immune function and its adaptive significance.

First, I applied phylogenetically-informed comparative models to immunological data from around one hundred bird species. These analyses revealed substantial interspecific variation in innate humoral immune indices, with body mass and some ecological variables explaining part of this variation. Evolutionary models further suggested that both indices probably evolved under a combination of fluctuating selection and genetic drift..

Second, in a common garden experiment on barnacle geese, I examined the effects of seasonality, population origin, and physical activity on immune function. Immune traits showed clear seasonal variation, while the impact of physical activity was more complex and dependent on season and exercise experiences. These findings show that immune defences in migratory birds are shaped by seasonal rhythms, energetic demands and exercise experiences.

Third, in a wild black grouse population, I examined sex differences and viral infection in relation to immune function. Haemagglutination, reflecting innate humoral defences, was higher in females, consistent with greater investment in self-maintenance. By contrast, haptoglobin, a marker of stress and inflammation, was higher in males, possibly reflecting the costs of intense reproductive competition. Birds infected with Sindbis virus showed reduced haemagglutination and elevated haptoglobin, providing the first evidence of its negative physiological impact on this species.

Finally, I investigated the relationships between male immune traits and signalling traits in black grouse. Haemagglutination was positively associated with lyre length and fighting frequency, supporting the honest signalling hypothesis. By contrast, haptoglobin levels increased with lek attendance, suggesting potential physiological costs of reproductive effort. These findings indicate that different immune indices relate to signal traits in distinct ways: some reflecting male quality, others highlighting the costs of reproductive investment.

Taken together, this thesis demonstrates that avian immune defences are shaped by complex trade-offs across ecological, behavioural, and physiological contexts. Immune function is not simply a mechanism for resisting pathogens but rather a dynamic adaptive system, closely linked with migration, reproduction, and sexual selection. By combining comparative, experimental, and correlative approaches, ranging from large-scale phylogenetic analyses to controlled experiments and field studies, this work contributes to a deeper understanding of how animals adjust immune function in relation to ecological challenges and reproductive investment, and it refines the theoretical framework of ecological immunology.