Chicken despawning

I have a few thoughts that may help with this:

1. You have your chickens contained, right? If an animal can wander more than 20 blocks, they will despawn. So, make your animal pens no more than 10-12 interior blocks wide/deep. Realistically, for chickens I never use a pen larger than a 5 x 5 interior
space, but I also make automatic chicken machines that require a small space.

2. Are you in/near a jungle biome? Ocelots may be eating your chickens. Since they can spawn in any light level, this is hard to prevent. Try surrounding the enclosure with water, as nothing can spawn on water. I discovered this after building in the
jungle and returning to find an empty enclosure with nothing but a very happy ocelot inside.

I had the same problem with sheep on my previous world now my chickens are despawning, I started with two chickens, I made them breed about twice and threw all my eggs in the pen and ended up with about 10 chickens. I went away for a little while and when
I came back there was not a single chicken to be seen.

I think you need to kill every wolf you see, since they will never despawn. My world is 99.8% water, so I don't have a problem with wolves. I'd be happy if I could just get something other than chickens and squid to spawn.

Bone meal, of course, is optional. I simply worked on other things while the first several rounds of pumpkins developed. Ten or so generations in and I had so many planted seeds that pumpkins were popping out in the my farm quicker than I could harvest
(and then destroy their mature stems for seeds). As for the chicken menace, after a couple of chickens found their way into my melon farm and destroyed crops, I managed to get all 8 chickens that spawned in my main base in a pen where they are now unlikely
to be causing any more problems (since passive mobs no longer despawn).

Apparently animals despawn if they are able to move more than 20 blocks one direction.

(21x21 pen = animals despawning).

Which I'm perfectly ok with..