For two consecutive years 24 hours or more of continuous precipitation during early June resulted in mortality in nestling Tree Swallows (Iridoprocne bicolor) and Cliff Swallows (Petrochelidon pyrrhonota) in Marin County, California. There have been few reports of swallow nestling mortality caused by inclement weather in North America. Chapman (1955) reported abandoned eggs and death of nestling Tree Swallows at Princeton, Massachusetts in 1940. Paynter (1954) reported Tree Swallow nests with abandoned eggs due to a storm on 8-9 June at Kent Island, Wisconsin. Drury (1959) reported mass nestling mortality due to continuous rain from 12-20 June in Massachusetts.