Gave bats the feral trait, capping their defense in any village to 40%.

data/core/macros/traits.cfg

@@ -166,6 +166,18 @@
     [/trait]
 #enddef
 
+#define TRAIT_FERAL_MUSTHAVE
+    # Units with trait Feral only receive 40% defense in villages regardless of
+    # the base terrain the village is on.
+    [trait]
+        id=feral
+        availability="musthave"
+        male_name= _ "feral"
+        female_name= _ "female^feral"
+        description= _ "Receive only 40% defense in villages"
+    [/trait]
+#enddef
+
 #define TRAIT_WEAK
     # Units with trait Weak get a -1 increment in hitpoints and melee damage.
     [trait]

data/core/units/bats/Bat_Blood.cfg

@@ -7,6 +7,7 @@
     profile="portraits/monsters/bat.png"
     {MAGENTA_IS_THE_TEAM_COLOR}
     ellipse="misc/ellipse"
+    {TRAIT_FERAL_MUSTHAVE}
     hitpoints=27
     movement_type=smallfly
     movement=9
@@ -31,12 +32,13 @@
         mirror="true"
         image="portraits/monsters/transparent/bat.png"
     [/portrait]
-    [defense]
-        village=60
-    [/defense]
+
     [resistance]
         cold=70
     [/resistance]
+    [defense]
+        village=-60
+    [/defense]
 
     [defend]
         direction=s,sw,se

data/core/units/bats/Bat_Dread.cfg

@@ -6,23 +6,18 @@
     image="units/undead/dreadbat-se-3.png"
     profile="portraits/monsters/bat.png"
     {MAGENTA_IS_THE_TEAM_COLOR}
+    {TRAIT_FERAL_MUSTHAVE}
     hitpoints=33
     movement_type=smallfly
-    [defense]
-        village=60
-    [/defense]
-    [resistance]
-        cold=70
-    [/resistance]
     movement=9
     experience=150
     level=2
     alignment=chaotic
     advances_to=null
     {AMLA_DEFAULT}
+    undead_variation=bat
     cost=34
     usage=scout
-    undead_variation=bat
     description= _ "The most vicious, predatory, and successful of the Blood Bats become Dread Bats, gaining the ability to drain not merely the blood but the very life energy of their victims."+{SPECIAL_NOTES}+{SPECIAL_NOTES_DRAIN}
     die_sound=bat-flapping.wav
     [portrait]
@@ -37,6 +32,14 @@
         mirror="true"
         image="portraits/monsters/transparent/bat.png"
     [/portrait]
+
+    [resistance]
+        cold=70
+    [/resistance]
+    [defense]
+        village=-60
+    [/defense]
+
     [attack]
         name=fangs
         description= _"fangs"

data/core/units/bats/Bat_Vampire.cfg

@@ -7,6 +7,7 @@
     profile="portraits/monsters/bat.png"
     {MAGENTA_IS_THE_TEAM_COLOR}
     ellipse="misc/ellipse-nozoc"
+    {TRAIT_FERAL_MUSTHAVE}
     hitpoints=16
     movement_type=smallfly
     movement=8
@@ -31,12 +32,13 @@
         mirror="true"
         image="portraits/monsters/transparent/bat.png"
     [/portrait]
-    [defense]
-        village=60
-    [/defense]
+
     [resistance]
         cold=70
     [/resistance]
+    [defense]
+        village=-60
+    [/defense]
 
     [defend]
         direction=s,sw,se