Night-migrating birds often accumulate near bright man-made light on nights with low cloud cover or rain. Mass avian mortality events associated with this phenomenon have been documented for more than 150 years. Understanding the mechanism that induces the aggregation of migrants in lighted airspace could lead to a reduction in such mortality. Toward this end, we subjected night-migrating birds flying in dense cloud cover to alternating short periods of different artificial light characteristics. Bird aggregation occurred during periods of white, blue, and green light but not in red light or flashing white light. We discuss these results with respect to visual and magnetoreception-based aggregation theories and the phenomenon of light-induced bird mortality at tall television towers in North America.