according to info on uoguide.com you have a 10% shot of it becoming a battle lizzard if you remember to water egg daily. 5 % if you missed one day. 0 if you missed two (or they burn, anyway you wont get it then)
Thats for each egg.
Also, I killed 34 last night, and I gess, 50ish the days before (minumum 50 at least) and 29 so far today.
Total Kills = minumum of 113 chickens ( should be more)
Eggs = 1
so the chance of getting a battlechick with my luck is small!
Remember to feed any chicken you get when you get it, I belive when fed the day after 1st feeding, it will have a chance of laying an egg.
if its random or 100% I dunno...