1. Note that "those claiming PIWO and other woodpecker species make double-knocks" now includes Cornell (supporting link here).
2. More double-knocky background reading is available here and here.
3. Please also note this paragraph from Cornell's '05-'06 final report:
Ducks—During this past field season, we have learned that under certain circumstances duck wings can produce a double popping sound similar in timing to the double knocks of Campephilus woodpeckers. The mechanism by which ducks produce these sounds is unclear. We suspect that these pops are somehow mechanically produced by the ducks’ flapping wings, rather than being vocally produced. We hypothesize that a double knock from a nearby woodpecker would be distinguishable from a duck-generated double pop by a loud, resonant, “woody” quality. However, many of the double knocks that we have detected sound more distant and lack an unmistakable woody sound, making exclusion of ducks difficult in those cases. Based on the presence of duck vocalization and/or “wing-whirring” we have exculded 57 double-knock-like sounds from future IBWO analyses.