minecraft-src/data/minecraft/chicken_variant/warm.json

{
  "asset_id": "minecraft:entity/chicken/warm_chicken",
  "spawn_conditions": [
    {
      "condition": {
        "type": "minecraft:biome",
        "biomes": "#minecraft:spawns_warm_variant_farm_animals"
      },
      "priority": 1
    }
  ]
}