Studying life history strategies is crucial for understanding the diversity among organisms. The trade-offs in traits related to survival, self-maintenance and reproduction play an important role in the variation of life histories and can be adjusted in response to environmental and physiological factors. Telomeres, which are repetitive DNA sequences found at the ends of chromosomes, can serve as molecular markers linking these factors and ultimately fitness. Telomeres naturally shorten with age and stress, such as diseases, and when they become too short, they trigger cellular senescence and ageing. However, some telomere restoration mechanisms can counteract this process. In this thesis, first I have adopted a theoretical approach to review the telomere research field and develop hypotheses for future exploration. Then, I also included an empirical section that aimed to evaluate and test some of these hypotheses' general patterns using data from a wild population of the polygynous bird species, the great reed warbler (Acrocephalus arundinaceus). I have taken advantage of the longitudinal study conducted for 40 years at Lake Kvismaren that have collected a comprehensive dataset on life history, behaviours and fitness components, along with a sampling of all the bird born and breeding in the area. I measured telomere length using the quantitative PCR method, and to investigate the ecological and evolutionary implications of parasites, I used the multiplex PCR method to detect the presence of avian malaria parasites in the birds' blood. In the first part of this thesis, I and my collaborators present a summary of telomere hypotheses that apply to the fields of ecology and evolution. We also classify and group these hypotheses based on their research context and hierarchical similarities. Furthermore, we identify gaps in the knowledge, such as the lack of hypotheses that can clarify telomere elongation patterns in the wild. As a result, we propose and expand on a novel hypothesis that highlights the significance of elongation patterns and encourages further exploration in this area of telomere dynamics. The second section of the thesis aimed to test some of the assumptions made in the previous hypotheses. The studies were focused on examining the existence of a lower threshold for telomere length and I revealed that individuals with critically short telomeres disappeared from the population at a greater rate than those with longer telomeres. Furthermore, life stressors such as infections had an impact on these selection patterns. In this thesis, I also found that even in early-life a considerable percentage of individuals can display a net increase in telomere length (telomere elongation), and this prospect was dependent on factors like infection and the current telomere length. Finally, this thesis demonstrates that various factors including age, malaria status, and harem size can predict the dynamics of telomere length in male and female great reed warblers. These sex-specific telomere dynamics could potentially provide indications of the individual's phenotypic quality. In summary, this thesis has contributed to our understanding of telomere dynamics in the wild, particularly concerning the complex interactions between telomere length, infections and life history trade-offs. It has addressed some gaps in our knowledge of telomere biology and provided insights into important yet under-explored areas, like telomere elongation patterns. The thesis highlights the need for further research into telomere elongation events, the impact of short telomeres on individual life histories, and the potential use of telomeres as a biomarker for individual quality or as a measure of environmental stressors faced.