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I had hoped to write a followup to my prior mention of the lengthy Hannah Hunter dissertation on acoustic evidence, but it would just take too much time to do properly. So, instead, for those who may have found the piece too long to read through, I’ll just leave here a few key verbatim passages from the paper (a small sampling of all Hannah has written), which hit upon some of the issues as to why I can't take most auditory evidence too seriously [I have bolded several lines for emphasis]:
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“After the 2005–06 field seasons, however, digital playback was de-emphasized as a search method in Cornell’s searches. It had a relatively small ‘carrying distance’ (Rohrbaugh et al., 2007, p. 36), so was not particularly helpful in sampling large, dense areas…. A more geographically appropriate ivorybill communication tool was found in mechanical double-knock mimicry. This method can be traced back to Tanner, who wrote that ‘pound[ing] loudly on a wooden stub’ to imitate a double-knock would ‘sometimes make the bird answer by calling or rapping itself’ (Tanner, 1942, p. 42). Additionally, double-knock simulation has been successful in communicating with other Campephilus woodpeckers in South America.”
“Sonic communication methods, however, are not embraced by everyone. Hill, for instance, wrote these instructions to future ivorybill seekers:
[I]mitations of ivory-bill calls and knocks should absolutely never be used in the Choctawhatchee River basin. … There is no evidence that such sounds have any positive effect on your chances of seeing an ivorybill but such sounds will corrupt our monitoring efforts and will mislead other birders into thinking they have detected an ivorybill. … I think we can all agree that when we hear a kent call or a double knock in the forests … we want to be confident that it is an ivorybill and not a human imitating an ivorybill.”
“This passage encompasses several common criticisms of sound-making methods. Firstly, that they have not been proven to ‘work’. Contrary to the idea that ivorybills would be lured in or would respond to sonic mimicry, some believe that ivorybills are territorial birds that could be scared away from an area if they hear the apparent calls of other ivorybills. Some have even speculated that Cornell’s use of active sonic methods in the 2000s drove away what may have been an active ivorybill population from the search areas (M. A. Michaels, personal communication). Many of these concerns stem from insecurity about how little is known about ivorybill communication and behavior, and fear that what is true for other Campephilus species in South America might not be true for ivorybills—and, indeed, that what worked with the ivorybills Tanner encountered in the 1930s might not garner the same response in ivorybills today. 7 Courtman, for instance, has recently stopped using active sonic methods altogether, since he is ‘just not sure of what we’re communicating’ (M. Courtman, personal communication).
The other concern Hill highlights is that sound-making methods might ‘corrupt’ by misleading other searchers.”
“Despite these processes, Charif stressed that even the strongest recordings from White River weren’t ‘proof of anything’, but instead ‘very intriguing evidence’ (R. Charif, personal communication). One issue for the persuasiveness of these putative ivorybill recordings is that there is such limited information about ivorybill sounds. There is only one undisputed sound recording series to compare against: one that is rather short, controversial, and difficult to replicate. As Michaels told me, ‘the [1935] recordings from the Singer Tract are like a Hollywood movie … so everybody, the public expectation, and even the scientific expectation, is for something that’s impossible to obtain in the real world’ (M. A. Michaels, personal communication). The parabolic reflector used by the 1935 recordists (Figure 2) is ‘like an incredible zoom lens .… That recording was made at a nest tree so they were very close, [and] they had a super high gain microphone. … the recordings that you pick up from distant birds [with ARUs] are nothing like that’ (R. Charif, personal communication). This is largely an issue of signal-to-noise ratio (SNR): the relative strength of the desired signal (here, the ivorybill sounds) against background noise. ARUs, however, by design pick up sounds indiscriminately: they capture soundscapes with lots of information (other species, weather, human presence, etc.), which may or may not contain ivorybills far away or in passing. Any surviving ivorybills included in these sonic captures, then, would likely sound and look (on a spectrogram) quite different than the 1935 recordings (A. Warde, personal communication).”
“Many involved in recent searches initially thought kent calls to be distinctive enough that sound-alikes wouldn’t be an issue: ‘if you were to say to anybody who knows North American birds … who has listened to the [1935] Allen-Kellogg recordings, like “suppose I tell you that you could confuse a Great Blue Heron for an Ivory-billed Woodpecker”, they would think you were absolutely out of your mind’ (R. Charif, personal communication). Part of the problem, Charif says, is that ‘when you have tens of thousands of hours of recordings, you also get recordings of very unusual anomalous sounds, which are not the typical sounds of that species’. Additionally, though ivorybill sounds are somewhat distinctive, the acoustic structure of kent calls and especially double-knocks are relatively simple, and thus vulnerable to spectrogram look-alikes (M. Lammertink, personal communication). When combined with the SNR issue, false detection of ivorybill sounds was a real concern, and sound-alikes continue to be a common retort against acoustic evidence.”
“The authors argued that Gadwall ducks had been recorded in other contexts to make such sounds, and that their vocalizations were audible in some of Cornell’s putative ivorybill recordings (Jones et al., 2007). For all these reasons, a close quantitative match between an ARU recording and the template sounds would not necessarily mean the recording was an ivorybill. Indeed, the process of translating sounds into recordings, and then into spectrograms, is not totally immutable—not only because background and contextual sounds around specific events might be lost, but also because field recording conditions and spectrogram processing choices affect the final appearance of the image. In other words, this is translation with corruption, in subtle but significant ways.”
“This is not to say that sound can never offer consequential evidence in science,9 but rather that, in this case, the physical and conceptual geographies of ivorybill searching limit sound’s persuasiveness. In addition to the issues outlined above, there are the limits to the contemporary acoustic evidence itself: Many of the ARU recordings not only have weak SNR but are also rare and generally brief. Both Mennill and Lammertink, who have studied other Campephilus species in Central and South America, pointed out that the ivorybill-like captures of the Cornell and Auburn-Windsor searches were atypical of these kinds of birds: ‘whatever is producing those sounds that we’ve got on the Choctawhatchee is not producing the double-knock and the kent sounds very often … if this is an Ivory-billed Woodpecker, why isn’t it double-knocking 100 times in a morning? Why are we getting 5 or 10 double-knocks in a day and not many, many more than that?’ (D. Mennill, personal communication). Although there are several contemporary recordings that these scientists still find intriguing, all the issues discussed in this section make it unlikely that the conservation community will be convinced of ivorybill survival from sound recordings alone, or even in context with other forms of evidence. This is especially true given widespread ornithological skepticism about ivorybills: In such a beyond-belief, high-stakes case, there is little room for ambiguity. The capture, translation, and analysis of field sound recordings are thus of most use when circulated back to the field to inform searchers’ navigation. That is to say, in their role of expanding and quantifying the listening geographies of search sites towards the goal of an irrefutable video or photograph of an ivory-billed woodpecker.”
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Again, Hannah's full paper is HERE.
....I suspect I may have one more post before year's end, but in case not, a safe, healthy New Year ahead to all!
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