Avian influenza virus (AIV) and avian paramyxovirus-1 (APMV-1) are two economically important pathogens whose epidemiology is complicated by a broad avian host range, a global distribution, and varying levels of virulence. Phylogenetic methods have been developed that enable the incorporation of host and location sampling data into evolutionary models, allowing the inference of viral spread among hosts and regions. Understanding the factors that drive APMV-1 and AIV movement around the world and between species may provide important information for outbreak control and prevention. This dissertation presents three analyses that use discrete trait diffusion models based in a Bayesian phylogenetic framework to investigate viral diffusion of AIV and APMV-1 among avian hosts and geographic regions. In each manuscript, viral sequence data is used as the foundation to infer both the pathogens evolutionary history as well as its diffusion history. First, a global sample of APMV-1 complete fusion gene sequences provides the framework quantify the dispersal of this virus around the globe and between avian host orders. In addition, to investigate the impact demographic, evolutionary and ecological factors have on AIV diffusion, two contrasting North American AIV scenarios are examined: enzootic AIV dynamics within wild birds and epizootic dynamics within domestic poultry observed during the 2014-2015 highly pathogenic avian influenza (HPAI) H5N2 outbreak. All three manuscripts demonstrate viral diversity is influenced by both host and location, suggesting avian populations should not be treated as homogenous, but rather are structured by host types and geographic barriers. This is further supported by the generalized linear models constructed to evaluate ecological and environmental predictors of AIV dispersal. Both enzootic and epizootic AIV models provide evidence that distance between geographic regions acts as a strong barrier to transmission. In all, these manuscripts provide critical information for future APMV-1 and AIV outbreak prevention and control by quantifying viral movement among host types and between geographic locations. Further, ecological factors identified as associated with AIV movement in both low and highly pathogenic viruses may be helpful to inform AIV surveillance plans.