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Spectrographically, the "classic" Ivorybill "kent" as documented on the old recordings is a very simple sound. It's just a series of harmonics at 700Hz intervals with essentially no variation in pitch during an individual call; which harmonic is the loudest will depend on conditions and recording equipment. It is just a "toot" such as can be made by many animals or musical instruments. On the clean (old)recordings the "k" and "t" are visible as teeny subtle clicks beginning and ending each note. These vanish rapidly with any slight degradation of the sound. I believe everyone who has tried to analyze the IBWO calls has noted how plain they are on the spectrograms, sort of an Orange-crowned Warbler of avian "honks." But, like an OCWA there possibly are distinguishing characters, and there's also the issue of context: A Yellow-crowned Night Heron may produce a single "ahnk" that looks similar on the spectrogram, but will it produce a staccato series of flat pitch and uniform duration? I dunno. The same applies to mimicry. We've all been fooled momentarily by "Eastern Wood Starling-Wees" and "Carolina Mocking-Wrens," but the context rapidly eliminates our confusion most of the time. It is also well known that trained ears can make distinctions that songrams fail to reveal; but there we get into subjectivity and the fact that the vast majority of living birders have never actually heard an in-the-wild-for-sure-no-doubt-about-it Ivorybill "kent" to use for training our ears.
Two real points here:
1. Publication of detailed analyses of real IBWO calls, ARU sounds, and confusion sounds is essential for understanding any of this better; we can hope this will be forthcoming very soon!
2. Even if fairly reliable ARU signatures can be identified, they're almost surely going to remain secondary evidence, useful for tracking birds whose existence has been concretely established by other means, or for suggesting the possible presence of birds that need to be searched for and confirmed. They may have a strong effect on the conscience and confidence of a given individual birder (like me), but definitively reestablishing the species as extant on State or global lists is a whole nuther matter.
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