At a marsh in Okarito I felt fortunate to be able to sit on a boardwalk at sunset and listen to the sound of at least a dozen Fernbirds calling and singing back and forth. The main call/song of the Fernbird is a series of double notes with an odd echoing quality. The best description I have heard was from Carol Davies of Birders' Rest who compared the quality of noise to two rocks hitting together.
Standing on the boardwalk there were several birds calling from within a few feet of me, though I didn't get even a glimpse for a while. The water in the marsh was high and the reeds, being short, only poked a couple feet above the glistening black liquid in the fast dimming light. This however is typical Fernbird habitat and they have adapted very well to it. As I tracked one calling individual in the reeds I finally caught a glimpse just as it dropped out of view. I followed rustling reeds with my eyes as the bird moved across the marsh towards another calling bird. It came so close as to be almost under my feet. The dim light gave little away although I could just make out the bird deftly moving through the rushes as if it was crawling on solid ground. Using its toes with a precision similar to a parrot the Fernbird seemed to glide inches above the water, gripping each subsequent clump of reeds as it came into reach. Soon it was out of view and the rustling was no longer visible as it moved into the larger plants of New Zealand Flax at the edge of the marsh.
If by chance you are lucky enough to get a good look at one, usually achieved by calling one in with an imitation of their call, you will see a small beautifully brown-streaked bird with a warbler-like bill and a long ragged tail.