Ivory-bill Double Knock Sound-Alikes
On this page, you can listen to other known sounds similar to double knocks of Campephilus woodpeckers.
On rare occasions, drumming Pileated Woodpeckers produce double knocks that are indistinguishable from double knocks of Campephilus woodpeckers. In most cases, these can be identified by their proximity to typical pileated drum rolls.
Here is a 19-second recording made by an ARU on a spring morning in the White River National Wildlife Refuge. Approximately 1.5 seconds into the recording, there is a typical loud drum roll from a woodpecker (likely a Red-bellied Woodpecker); then at about 17 seconds into the recording, there is a loud double knock from the same bird. Other drumming woodpeckers are audible in the background.
Close examination of the double knock (lower panel) shows that it is virtually identical to the first two strikes of the drum roll (upper panel):
On several occasions, members of the Cornell Lab of Ornithology ivory-bill search teams in Arkansas have heard faint double-knock-like sounds that appear to come from ducks in flight. We have found numerous instances in our ARU recordings of double-knock-like sounds closely associated with wing-generated sounds of flying ducks (wing-whir) and duck vocalizations.
Click on the play arrow below the spectrogram to hear an 18-second recording that includes two double-knocks among wing and vocal sounds of ducks.
Although we have not recorded double-knock-like sounds from ducks when an observer is present, we strongly suspect that under certain circumstances some ducks produce double wing claps that could be mistaken, particularly in recordings, for distant Campephilus double knocks.
As with the recording presented here, many recordings of double knock sounds generated by duck wings include quacks and wing whirring. The presence of these sounds are an immediate red flag to acoustic analysts. However, not all suspected duck-wing double knocks include such clues, so we must rely on our ears to discern the non-resonant duck wings from more resonant (woody) sounding woodpecker drums.
We are working on a thorough analysis of duck-wing sounds so that they can be more readily identified and removed from our sound review process. To read more about the similarities between double knocks and mechanical wing sounds produced by ducks please see: Jones, Clark C., Troy, Jeff R., Pomara, Lars Y., "Similarities Between Campephilus Woodpecker Double Raps and Mechanical Sounds Produced by Duck Flocks," The Wilson Journal of Ornithology, 119(2): 259-262, 2007.
In much of the Arkansas Big Woods ivory-bill search area, gunshots are common at certain times of the year, because this is a very popular duck hunting area. This raises the question of whether gunshots could be confused with double knocks from a woodpecker. In order to be considered a potential double knock, a sound must contain two distinct strikes, between 60 and 120 milliseconds apart, clearly separable by ear and in the spectrogram.
In March 2006, Cornell Lab of Ornithology field biologists in Arkansas used ARUs to record the sounds of three weapons (a .22 caliber rifle, a .30 caliber rifle, and a .12 gauge shotgun) fired twice in close succession at distances between 200 and 2000 meters, in 200-meter increments, in habitat similar to the areas being searched for ivory-bills. For safety reasons, the interval between the two shots was longer than in a real woodpecker double knock. None of the resulting recordings would have been mistaken for a woodpecker in our review and analysis protocol.
Shots that were close to the recorder (less than 400 meters for the .22 caliber rifle, less than 1400 meters for the .30 caliber rifle, or less than 1000 meters for the shotgun) overloaded the recorder, unlike any naturally-occurring sound, other than thunder. At greater distances, the two shots were invariably badly "smeared" on the spectrogram by reverberation in the forest environment. The examples below compare known woodpecker double knocks and several of these gunshot tests.