What are fiddlehead ferns exactly?

Fiddlehead ferns grow in mysterious places along the banks of rivers and streams. People who fancy them in cooking usually don’t tell anybody where they’ve found them, as they are so precious. The fiddlehead, so-called because it looks like the tuning end of a fiddle, is the very top of a young ostrich fern, still tightly furled and sheathed in a covering that can be a real pain to remove. To eat them, they must be picked before unfurling. Otherwise the leaves are poisonous. They make great pickles and a delicious vegetable side dish.