I’ve seen this story attributed mostly to an old fable by Aesop, but also have seen it attributed to the Native Americans or even the ancient Chinese. Regardless, the following is a particularly good version of it:
Many years ago there was a great battle between the animals. The mammals on the ground and the birds in the sky began a terrible war that lasted for many weeks.
At first it seemed as though the birds would win. There were far more birds than mammals. They could swoop down and peck away at the mammals and then fly up out of harm’s way before the mammals could attack.
Bat flew up to the birds. They swooped towards him, ready to attack! But bat flapped his wings and said, “Look! I have wings just like you. Do not attack me for I wish to fight on your side.”
“What a wonderful choice I’ve made. With so many more birds than mammals I will be on the winning side for sure.”
But the mammals were not that easy to defeat. Although there were fewer of them, they had sharp teeth and claws and were much larger than the birds.
To make matters worse, Bat awoke to a very windy day. The wind made it difficult for the birds to control their attacks. Before long it was clear that the mammals had won that day’s battle.
That night after the miserable defeat, Bat decided enough was enough. He flew down and joined the mammals.
Bat opened his mouth wide to show his pointy teeth. “Wait,” he said, “I am one of you! See, I have teeth in my mouth like a mammal — not a beak like a bird.”
The mammals pondered for a few moments and then agreed that Bat did indeed belong on their side.
The next day, the battle was again fierce. But this time the wind was gentle and the birds were able to organize. They pecked at the eyes of the mammals and flew away before they could be touched.
That evening, Bat snuck away from the mammal’s camp and quietly flew back up into the sky to join the triumphant birds. When they complained that they had seen him on the mammal’s side during the battle, Bat again flapped his wings and claimed to belong with the birds.
And so it went for many days. Whenever the birds won a battle, Bat would flap his wings and go with them. Whenever the mammals won a battle, Bat would bare his teeth and side with them.
Finally the animals grew tired of battling each day. The Chiefs held a meeting to make peace. They were all so exhausted, that it did not take long to reach a truce — but during the process the mammals and the birds began to talk about Bat.
“It doesn’t seem fair that he just switched sides whenever he wanted,” squawked Crow.
“No, not fair at all,” growled Bear, “Bat has wings but he did not stay with the birds.”
All of the animals nodded and looked at Bat, “Because you could not choose your friends during war, you will not have them during peace. From this point forward, you will only fly at night when everyone else is sleeping. You will have no friends among the mammals or the birds.”
And that is why Bat doesn’t have any friends.
On a possibly unrelated note, here’s a random screen shot image of a collection of quotes from people off of twitter – batty behavior indeed: