We document the spring 2007 die-off of 216 Tachycineta bicolor (Tree Swallow) in western New York due to a period of unseasonably warm temperatures followed immediately by a period of unseasonably cold temperatures. Dead swallows were collected from artificial nesting boxes, where they had roosted communally. A subsequent decline in nesting effort was detected during the following nesting season. Implications for future nest-box management are discussed.