minecraft-src/data/minecraft/worldgen/configured_feature/trees_birch.json

{
  "type": "minecraft:random_selector",
  "config": {
    "default": "minecraft:birch_bees_0002",
    "features": [
      {
        "chance": 0.0125,
        "feature": "minecraft:fallen_birch_tree"
      }
    ]
  }
}