Complex sentence: In Britain, Northern Fulmars historically bred on St. Kilda (where their harvesting for oil, feathers and meat was central to the islands' economy) and spread into northern Scotland in the 19th century, and to the rest of the United Kingdom by 1930.
Split into simpler sentences: In Britain, Northern Fulmars historically bred on St. Kilda (where their harvesting for oil, feathers and meat was central to the islands' economy). They spread into northern Scotland in the 19th century, and to the rest of the United Kingdom by 1930.