The Lower Eocene Green River deposits provide the earliest extensive insight into the North American post-Cretaceous avian radiation where a large variety of well-preserved taxa are available. It provides a unique insight into the early radiation of the galliform-anseriform complex and the radiation of early birds with perching adaptations. Comparison with faunas in Germany and England provide insight into the origin of the North American avifauna. Because of the rich environmental context available in the Green River sediments, it is possible to examine factors surrounding the evolution of modern avian diversity.