Bird mortality during nocturnal migration over Lake Michigan: A case study

Millions of birds die each year during migration. Most of this mortality goes unobserved and conditions surrounding the actual events are often not thoroughly documented. We present a case study of substantial migrant casualties along the shores of southwestern Lake Michigan during May 1996 when we found 2,981 dead birds of 114 species, mostly migrant passerines. An unusual sequence of events allowed us to document the circumstances surrounding this migratory bird kill. Bird carcasses appeared on the southwestern shores of Lake Michigan in the days following storm systems that produced high rain and in one case, hail. Encounters between birds and precipitation over open water were recorded by weather radar, and were followed by winds that drifted dead birds toward highly populated shorelines where the kill was observed and documented. Climatologically, May 1996 was exceptional for producing weather conditions that both killed birds en masse and allowed the mortality to be documented. As a result, this is one of the more thoroughly documented instances of a weather-related mass mortality event during migration.