Soo.. The only terrain mods I have in a modpack are WWOO, WWEE and Tectonic. Chickens seems to be spawning in caves, it's kinda funny because I'm there mining some ores and then I find a chicken lol. (sorry if there's any grammar issues) I have 214 mods, if it's useful for anything. :p

LO doesn't actually change any spawn rates, since O-Animals just replace vanilla ones that spawn in the world.

You may be finding chickens without their jockeys, or maybe another mod is spawning vanilla chickens in places they shouldn't be and LO is just replacing them :)