So Aric's Expansion Mod adds a bunch of new races, but some of them are not encounterable in combat, namely Halfkin Birds. But any time I try to add Bird to the Races list of any region, the game crashes every time I try to go outside.

Please use the Debug mod (https://itch.io/t/1137280/debugmod-v10d) to get the error messages as that will better narrow down the source of your problem. The Debug mod will have the side effect of stopping the game from crashing, but your problems will not be gone. Note that if you are making your changes directly to the files in the program folder then those changes will be erased if you press Apply or Reset in the Mods menu.