Lv. 2 Pidgey              
HP   -> 13 if 0-19 IV
        14 if 20-31 IV

Def  -> 5 if -Def 0-19 IV
        6 if -Def 20-31 IV, 0-19 IV Neutral or +Def
        7 if 20-31 IV Neutral or +Def
Rolls
 - 5 Def  ->  6 (3/16)    = 72.4% at both
              7 (12/16)   = 78.4% at both
              8 (1/16)    = 78.4% at both
 - 6 Def  ->  5 (1/16)    =
              6 (14/16)   = 72.4% at both
              7 (1/16)    = 78.4% at both
 - 7 Def  ->  5 (15/16)   = 62.4% at both
              6 (1/16)    = 72.4% at both
 
Lv. 3 Pidgey 
HP   -> 15 if 0-19 IV
        16 if 20-31 IV
Def  -> 6 if -Def 0-19 IV
        7 if -Def 20-31 IV, 0-19 IV Neutral or +Def
        8 if 20-31 IV Neutral or +Def
Rolls
 - 6 Def  ->  5 (1/16)    = 62.4% at both
              6 (14/16)   = 62.4% at both (Second hit takes to 91.3% or 99.9%)
              7 (1/16)    = 78.4% at 15 HP, 72.4% at 16 HP
 - 7 Def  ->  5 (15/16)   = 62.4% at both (Second hit takes to 91.3% if 15 HP, not worth if 16 HP)
              6 (1/16)    = 62.4% at both
 - 8 Def* ->  4 (15/16)   = 54.8% at 15 HP, 50.3% at 16 HP
              5 (1/16)    = 62.4% at both (Second hit goes to 78.4%)
*If 12 ATK gets same rolls as 7 Def

Lv. 4 Pidgey 
HP   -> 17 if 0-19 IV
        18 if 20-31 IV
Def  -> 7 if -Def 0-19 IV
        8 if -Def 20-31 IV, 0-19 IV Neutral or +Def
        9 if 20-31 IV Neutral or +Def
Rolls
 - 7 Def  ->  5 (15/16)   = 62.4% at both
              6 (1/16)    = 62.4% at both
 - 8 Def* ->  4 (15/16)   = 50.3% at both
              5 (1/16)    = 62.4% at both
*If 12 ATK gets same rolls as 7 Def
 - 9 Def ->   4 (15/16)   = 50.3% at both (Second hit takes to 78.4% if 17 HP, 72.4% if 18 HP)
              5 (1/16)    = 62.4% at both