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After looking back at the text, I don't think we get a really detailed reason for his feathers growing in December.
"And yet he not only survived his worst winter, but seemed improved with the first sunny days. He remained motionless for several days in the farthest corner of the courtyard, where no one would see him, and at the beginning of December some large, stiff feathers began to grow on his wings, the feathers of a scarecrow, which looked more like another misfortune of decrepitude. But he must have known the reason for those changes, for he was quite careful that no one should notice them...."
From this section we can interpret that the angel knew he was molting and knew his feathers would come back fresh and strong. Even in his own decrepit state this would have given him hope, and thus he readied those refreshed wings for flight. This would tell us that this "molting and regrowth" was a regular part of his life....... similar to the molting of other birds or animals. Their cycles would be expected.
A Very Old Man With Enormous Wings