Take a look at table 4 (page 17) of CLO's '05-'06 search report here (PDF).
In just over 9,000 hours of ARU recordings, their preliminary report shows over 400,000 automated detections of potential IBWO sounds (over 240,000 potential vocalizations and over 160,000 double knocks).
That's an average of over 40 potential detections per hour!
Numbers like that help explain why you shouldn't get carried away if you hear some potential IBWO sounds anywhere in the woods (even if you're in an alleged "hot zone" and even if you've also glimpsed an intriguing bird within the hour).
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