As critical habitats for birds, the structure of forest ecosystems is not only a key factor in determining the spatial patterns of bird diversity and community composition, but also plays an important role in maintaining and enhancing the functions of forest ecosystems. Over the past few decades, global forest dynamics-characterized by varying degrees of degradation, restoration, deforestation, and afforestation-have resulted in significant and complex changes to forest structure. As a result, understanding how bird diversity responds to forest structure and its changes has become a central topic in the field of forest ecology and biodiversity conservation. In this study, we first review the key characteristics of forest structure and their heterogeneity, considering both horizontal and vertical dimensions. The definitions, calculation methods, and ecological significance are also summarized. Horizontal structure refers to the composition, configuration and distribution patterns of vegetation community, while vertical structure describes the bottom-to-top stratification of vegetation community such as the herbaceous layer, shrub layer, and canopy. Both aspects play critical roles in shaping habitat suitability and availability for avian species. We then explore the main data collection methods and commonly used research approaches for examining forest structure and bird diversity across plot, landscape, and regional scales. This study synthesizes the phenomena, patterns, and underlying mechanisms by which forest structure affects bird diversity at different scales. The findings indicate that forest structure can influence bird diversity by altering food resources, available habitats, microclimate conditions, and interspecies interactions. The life history traits and ecological characteristics of bird species play a decisive role in determining how they respond to changes in forest structure. Birds with specialized habitat requirements may be more sensitive to forest structural changes than generalist species. However, we find that the mechanisms by which forest structure influences biodiversity differ across scales, and the conclusions are subject to a certain degree of uncertainty. In addition to summarizing existing knowledge, this study also explores the potential of emerging observational techniques, such as citizen science, UAVs (unmanned aerial vehicles), and LiDAR (light detection and ranging), in advancing research on the relationship between forest structure and bird diversity. We also emphasize the importance of multi-scale comparative studies, as the relationship between forest structure characteristics and remote sensing-derived parameters, as well as the inferred influence mechanisms, exhibits strong scale dependence. We advocate for interdisciplinary collaboration among ecologists, conservation biologists, and policymakers to address complex conservation challenges, thereby providing scientific insights for forest management and biodiversity conservation.