[Level 1 Field Mouse]
/setblock ~ ~ ~ minecraft:mob_spawner 0 replace {EntityId:Silverfish,SpawnData:{CustomName:"Field Mouse [Lvl. 1]",CustomNameVisible:1}}
[Level 2 Rat]
/setblock ~ ~ ~ minecraft:mob_spawner 0 replace {EntityId:Silverfish,SpawnData:{CustomName:"Rat [Lvl. 1]",CustomNameVisible:1},Delay:500}