Volumetric velocimetry is used to investigate the evolution of tip vortices in the wake of a model wind turbine. The power spectral density of streamwise velocity fluctuations is estimated at several downstream locations, and changes in the magnitude of the frequency peaks corresponding to the blade-pass and turbine rotational frequency are observed. Full volumetric visualization of the coherent structures depicts the vortex merging process responsible for the change in detected frequency peaks, capturing the physical mechanism likely responsible for many similar previous observations.