Charif on the recorded double-knocks

On Russ Charif's treatment of the recorded double-knocks in his AOU presentation:

I completely agree with one of his opening quotes: "Because of the simple structure of these sounds, it can be very hard to definitively rule out other sources for any particular double knock."

He put up a slide containing this information:
Many non-biological sources can produce closely spaced double knock-like sounds: tree branches in wind, gunshots, raindrops, vehicles and other machinery.

He then went on to point out that the 54 recorded double-knocks were not randomly distributed in space or time, implying that they were not all likely produced by non-biological sources. I can agree with that conclusion too.

However, disturbingly, he finished his presentation without even mentioning alternative *biological* sources of double-knocks--Pileated Woodpeckers do double-knock, and other woodpeckers and even American Crows are possibilities. Note that Jerome Jackson said: "I've heard pileated woodpeckers make that kind of sound; I've heard crows make that kind of sound in breaking open a nut."

It would be interesting to hear what other bird sounds were recorded within +/- five minutes of each of the recorded double-knocks. For example, I wonder if Pileated Woodpecker calls and/or drumming was recorded in conjunction with many of the double-knocks.