I had a batch of chickens escape when I updated to 1.8; that's ok. Some of the chickens got into a wide river, two swam almost all the way across. Since then I've logged out and in again, and teleported away and back too. Now I find the chickens can hardly move in the water. They don't drown, but they move so slightly in any direction that I thought they'd stopped moving altogether. (They turned and their wings flapped, but I couldn't see any other movement.) I tried to lead them with seed. They looked at me, but didn't move towards me. I was able to push them to shore easily, and once out of the water they were able to move well.
Another chicken I have a clearer view of (from a high bridge) seems to move about an eighth of a block in any direction, but then it stops. It bobs up and down very erratically, not bobbing for short periods. The chickens I tried to lead with seed did not bob at all while they were looking at the seed, so I suspect they only dip down when they start to move.
Edit: This slow motion starts the moment a chicken enters the water. (I wasn't clear on that when I initially wrote the report.)
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I tried a chick to see if MC-9388 might be related. I didn't find any direct evidence; the chick grew to adulthood in water.
One odd feature was the chick floated above the water, its feet only sometimes touching the water's surface. If this is because of the same motion problem, I can imagine a chick could get stuck lower down and drown.
galaxy_2alex
Duplicate of MC-48616
That chicken I can see from the bridge has moved at least 5 blocks since I filed the report. I see it moving in about 1/8th-block steps (maybe 1/4), repeating at about the interval it would take to swim one block in previous versions.
It seems the bobbing is unconnected to moving forward, or there is a random chance of attempts at forward motion working at all.