fix tileset issues with chicks

data/json/monsters/bird.json

@@ -78,7 +78,6 @@
     "material": [ "flesh" ],
     "symbol": "v",
     "color": "yellow",
-    "looks_like": "mon_chicken",
     "aggression": -99,
     "morale": -8,
     "melee_skill": 1,
@@ -110,6 +109,7 @@
     "name": { "str": "grey chick" },
     "description": "A tiny dull-colored chick, it could be from a number of different species.",
     "copy-from": "mon_generic_chick",
+    "looks_like": "mon_generic_chick_tiny",
     "volume": "7 ml",
     "weight": "5 g",
     "upgrades": { "age_grow": 14, "into": "mon_sparrow" },