How to convert the birds are flying in the sky to passive voice?

It is not easy (impossible?) to convert this sentence to passive voice.

The passive refers to something that happens to people/things.

# The fox ate the chicken, is the active voice. The subject (fox) did the action to the object (the chicken).

# The chicken was eaten by the fox. is the passive voice. The subject of the sentence, the chicken, was passive in the action. (well, it probably wasn't! However..) The subject (chicken) was being acted upon by the object of the sentence, the fox.

In the sentence in the question, the birds did nothing to the sky, and the sky had nothing done to it! The nearest approximation might be "The sky is being flown in by the birds!", but that hardly makes any sense!