U can change it with: DinoSpawnWeightMultipliers (https://ark.gamepedia.com/Server_Configuration) for the Dino Name Tag (https://ark.gamepedia.com/Creature_IDs).

So u need to put in the gameini:
DinoSpawnWeightMultipliers= (DinoNameTag=Owl,SpawnWeightMultiplier=7.0,OverrideSpawnLimitPercentage=true,SpawnLimitPercentage=0.50)

SpawnWeightMultiplier= how much the dino should spawn compared to other dinos
OverrideSpawnLimitPercentage= if u want to use the SpawnLimitPercentage
SpawnLimitPercentage= when the dino got a free spawn point (on all its spawn locations on the map) how often it will spawn (0.50= 50%)

im not sure how often the normal SpawnWeightMultiplier and SpawnLimitPercentage of the dinos are so u would need to test if the spawn of that dino is fine for u.

And yes u need to do it with every dino u want to change.