Currently successfully cross breeding two maxxed chickens only gives a regular chicken with no stats. I think it would be best if each stat of any two chickens being crossbred was averaged and then halved to give the stats of the child chicken.
Example:
Two chickens, Log chicken with stats 4/2/4, Flint chicken with stats 8/6/8. The average of each stat makes 6/4/6, half of each stat makes 3/2/3 resulting in a coal chicken with stats 3/2/3.

Probably because that's how breeding Chickens in the world works and it's what balances the mod.

@EmosewaGamer, correct, the breeder uses whatever logic normal chicken breeding uses.

See setycz/ChickensMod#76 and GenDeathrow/Hatchery#69