I google duck flying speeds just for fun. I feel speed is very tough to judge in the field and background, foreground vegetation and body size make for interesting illusions.

This looks like a decent discussion of the topic.
http://magazine.outdoornebraska.gov/2014/02/duck-speeds/

There are many more out there. Most discussions of speed are usually of top speeds which are probably not what we see when we are shooting at birds coming into decoys or flushing away, so pragmatically, I would guess we actually are shooting at velocities well under half of max.

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BrentD, (Professor - just for Stan)
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