The creature-spell 'chicken' is not really part of the game, but yes, I guess it would be an improvement if units with the 'never_chickens' property would be immune to it.
Are Avatars supposed to be affected by the creature spell but not the keeper spell? Is this a bug? Should the game be modified so Avatars are immune to the creature spell too? I'm not sure there's any point in making them immune to one but not the other, given that they seem to be otherwise identical...
The creature-spell 'chicken' is not really part of the game, but yes, I guess it would be an improvement if units with the 'never_chickens' property would be immune to it.