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Sick was born in Leipzig, Germany. He emigrated to Brazil in 1939.
A prominent ornithologist in Brazil, Sick published more than 200 papers, including his most influential work: Ornitologia Brasileira, Uma Introdução (1984), later translated into English as Birds in Brazil: A Natural History (1993). He also led scientific expeditions in remote areas in Brazil and described several species of birds, including the Brasília tapaculo, long-tailed cinclodes, Stresemann's bristlefront and golden-crowned manakin.