Charles Brown's research centers broadly on the behavioral and disease ecology of birds, with a specific emphasis on (1) the evolution of social behavior and (2) how arboviruses affect the ecology of birds. Most of his work has been with a single population of cliff swallows (Petrochelidon pyrrhonota), highly social birds that breed in large colonies throughout most of western North America. His long-term project (currently 27 years) at a field site in western Nebraska is among the longest running, continuous field studies on birds in North America, and the number of individuals marked (currently over 187,000 swallows) is the largest of any mark-recapture study of birds in the world. The cliff swallow project has sought to identify the causes of group living and to understand why breeding colonies vary in size. This has required measuring the costs and benefits of coloniality, which remains one of his major research emphases. Brown and his coworkers have investigated many of the major questions in behavioral ecology with cliff swallows, and we have used a variety of approaches. Their swallow work has included classical behavioral ecology observations and experiments, a large-scale mark-recapture project and associated demographic analyses, quantitative-genetic estimates of the heritability of behavioral traits, field endocrinological research on hormone levels, studies of selection, and analyses of alternative reproductive tactics including parentage studies. More recently, they have been studying how an RNA arbovirus, Buggy Creek virus (Togaviridae), affects the ecology of cliff swallows and house sparrows (Passer domesticus) that are associated with swallow colonies. Thus, while Brown works primarily on cliff swallows, his research is conceptually broad.