Hi.

Landed birds seem to move their heads in jerks, as if their vision would not work well in uniform motion; and at the same time, they are supposed to have good vision moving in flight...

Most walking birds keep the head motionless while the body/legs advance, until repositioning the head in advance and repeating. What is going on ?

You actually answered it - vision doesn't work in smooth movements. Even simply turning your head slowly is fast enough to cause a visual field to blur. For many animals, including us, the solution is called "saccades" - short, rapid eye movements which, during motion, "skip ahead", then fix, then "skip ahead" again, over and over, faster than we can detect, to minimize visual downtime.

 

Birds, for whatever reason, do not have saccades, and must instead perform the same motion our eyes perform with their whole heads. Several humans whose optic motor nerves have been damaged have compensated by developing this method, resulting in a very "bird-like" mannerism.