The factor responsible for the synchronous declines in eider nest numbers in 2008 in parts of the Baltic Sea must at least have been in effect during the time window spanning from the eider's departure from the wintering sites in March and April to the start of egg-laying at the end of April or beginning of May. The factor must also have been in effect not only at specific local sites but over large areas in the southern and central Baltic Sea but not in the northeastern Baltic Proper. To our knowledge, the most likely proximate factor fitting these criteria is the exceptional bloom of the potentially toxic P. polylepis in spring 2008.