In 1995, the Sri Lankan ornithologist Deepal Warakagoda heard a call he could not place in the rainforest at Kitulgala. It took six more years before he finally saw the bird that made it, in Sinharaja, in January 2001. It was described as a new species in 2004 and named Otus thilohoffmanni.
The forest had been watched for decades. The owl was there the whole time.
The Serendib Scops Owl is a useful thing to remember the next time a place feels fully known. Sinharaja was one of the most visited birding sites in the country, and something as large as a hand had gone unrecorded in it, because nobody had yet thought to follow that particular sound.