Scops owl
Scops owls voices sound like whistling or a row of high-pitched hoots, fewer than four hoots per second. The owls call like this for social interaction or to scare away other animals.  But different species of scops owls sound different from each other.  Human beings usually know scops owls are nearby because they hear them calling.  Human beings can tell scops owl species apart by their voices. Some, like the Serendib scops owl (Otus thilohoffmanni''), were discovered because birdcall experts could tell their voices were new to them.