Colorado’s first accepted record of a Cave Swallow (Petrochelidon fulva) involved an immature photographed at Prewitt Reservoir, Washington County, on 17 July 2013. The bird occurred amid this species’ substantial range expansion and a complex, rapidly changing pattern of vagrancy in the United States and Canada over the last 50 years. Here we detail the species’ colonization of the U.S. and summarize these patterns of vagrancy.