I summarize the 10-year composition dynamics of a riparian avifauna on the Arapaho National Wildlife Refuge, Colorado, USA. Only 15 of the 89 breeding bird species (19,393 individuals) were present each of the 10 years whereas 25 species were only recorded in a single year. Number of species in any given year ranged from 30 to 57 (34 to 64% of the 10-year cumulative species total). The number of species detected >1 and <10 years represented over half of all species with no pattern of either increasing or decreasing across years of study. Short-term definitions of species richness preclude the ability to define the major dynamic within an avian assemblage—species that use the site an intermediate number of years. Understanding the ecological prerequisites for these species should provide greater insight into current physical habitat and climate changes occurring on-site.