Anticipation of an action due to observation helps in capturing ducks in movement. This duck was giving me signs that it wanted to take off, therefore I immediately pre-focused to wait for that "decisive moment" . . .

Canon 5D Mark III with the 500L f4 lens and 1.4 Extender II handheld @ 1/800, f 5.6, ISO 640, + 1/3 exposure compensation. Cropped for composition purposes.

Cheers,

Mike