This thesis investigates the relationship between form and function of the wing musculature in birds and its implications for locomotion style. Modern birds show an extreme diversity of wing morphologies and flight modes, from ducks that constantly flap their wings, albatrosses and large birds of prey that fly by gliding and soaring to birds such as penguins and auks that use their wings for underwater propulsion. Previous studies in birds have investigated aspects of wing shape and forelimb skeletal morphology in relation to their flight modes and evolutionary history. However, the forelimb musculature has received less attention and our knowledge on its functional anatomy still has many gaps. In this work I aimed to address these knowledge gaps. First, I investigated the use of contrast-enhanced micro-computed tomography to visualise the soft tissue comprising the wing in a model species. I developed a workflow of digital dissection and presented a threedimensional (3D) model of a bird wing that can be used as a basis for future biomechanical analysis using computer modelling. Next I compared the muscle architecture of the wing musculature between closely related species of raptors and aquatic birds because they display a variety of flight styles. I provided quantitative data of muscle architecture for the entire musculature of the avian forelimb for a total of 25 species and found that birds showing similar flight mode showed similar patterns of muscle mass distribution along their wing. I also evaluated the range of movements that birds use during wing-driven locomotion in a kinematics analysis of swimming in the Humboldt using video-based markerless photogrammetry. I described the wing and body kinematics during a variety of diving modes and velocities, and provided values for hydrodynamics forces. This, in combination with numerical data of forelimb muscles, provided some insights into the role that individual muscles play during wing-propelled locomotion and its implication with a variety of flight style.