Bird populations around the globe are facing declines due to various factors, including climate change, pollution, and urbanization. Urbanization in particular presents a unique challenge, as various types of pollution impact reproductive success for a variety of bird species. In this study of Southern Sweden, we investigate the causes of hatching failure in urban and rural populations of Great tits (Parus major) and Blue tits (Cyanistes caeruleus). Through nest monitoring and egg dissections, we explore various causes of hatching failure, including abandonment, predation, and multiple types of embryo fatality. We hypothesized that there would be higher rates of hatching failure among the urban populations, with early embryo death being the leading factor in urban failure. We also predicted that predation would be the leading cause of hatching failure among rural populations.

Our results reveal significant differences in nest fate between urban and rural populations, with higher rates of abandonment observed in urban Great tit populations and predation primarily affecting rural nests. Despite hypothesized differences in causes of hatching failure, early embryo death emerges as a prominent factor across all populations. The study highlights the importance of understanding urbanization's impact on avian reproductive success. Recommendations for future research include continued monitoring and analysis to interpret the complex interactions between urbanization, pollution, and avian reproductive outcomes. Overall, this study contributes to our understanding of the ecological consequences of urbanization on bird populations and highlights the need for conservation efforts to protect avian biodiversity in urban environments.