The birds were at war with the beasts, and many battles were fought with varying success on either side.  The bat did not throw in his lot definitely with either party, but when things went well for the birds he was found fighting in their ranks; when on the other hand, the beasts got the upper hand, he was to be found among the beasts.  No one paid any attention to him while the war lasted.  But when it was over, and peace was restored, neither the birds nor the beasts would have anything to do with so double-faced a traitor, and so he remains to this day a solitary outcast from both.

Possible Mimetic Theory application: The birds and the beasts gain peace as they unite against the harmless bat.  How might that fable apply to our lives?