I have the same bug. Except I'm getting a lot more chickens since I've got a nether perimeter set-up but not complete. They spawn on the fortress and just around the nether. I have screenshots too if this needs more confirmed. They won't despawn right? So this could cause some problems.
I'm running on 1.7.4 on Mac OSX 10.6.8
Could be this related to why people are randomly finding chickens in the nether? The baby zombie pigmen are despawning and leaving the chickens?