behavior_packs/vanilla/spawn_rules/skeleton.json

{
  "format_version": "1.8.0",
  "minecraft:spawn_rules": {
    "description": {
      "identifier": "minecraft:skeleton",
      "population_control": "monster"
    },
    "conditions": [
      {
        "minecraft:spawns_on_surface": {
        },
  
        "minecraft:spawns_underground": {
        },

        "minecraft:brightness_filter": {
          "min": 0,
          "max": 7,
          "adjust_for_weather": true
        },
        
        "minecraft:difficulty_filter": {
          "min": "easy",
          "max": "hard"
        },

        "minecraft:weight": {
          "default": 80
        },
        
        "minecraft:herd": {
          "min_size": 1,
          "max_size": 2
        },

        "minecraft:biome_filter": [
          { "test": "has_biome_tag", "operator":"==", "value": "monster"},
          { "test": "has_biome_tag", "operator":"!=", "value": "frozen"}
        ]
      }
    ]
  }
}