Fiddleheads are actually the immature young curled-up tops of Ostrich fern, Matteuccia struthiopteris, so-called because the fronds look like ostrich feathers when full-grown.
I found these on the Long Trail ridge in central Vermont at about 2,500 feet elevation, in rocky soil among hemlock, beech and gray birch. They are 2 to 4 inches high, and the fiddle part is about 1 inch in diameter. They grow inside a fibrous brown casing, which you’re supposed to submerge in water and scrub off before eating. I tried some as they were, picked right from the trailside (which apparently you’re not supposed to do—carcinogens bah) and they were quite tasty, like a lemony spinach, though the brown stuff made them somewhat scratchy going down. I also had some for dinner the other night, steamed, then sauteed in butter and garlic. After cooking they lose some of the citrus flavor and become nuttier.
If you’re going to pick fiddleheads for eating, by the way, los eeenternets inform me that it’s best for the plants if you only pick 2 or 3 fiddles from each, so as not to damage the population for the future. You can see in my picture that there are five little fiddles in a bunch—apparently, underground they all come from one plant. I picked two, the tallest ones, and left the others alone.