As I haven't read it, the main characters are probably the birds. There might be a lead bird among them.
Most likely the birds are the main characters. There could be an older, wiser bird that leads the others. And perhaps there are some other animals in the barn like a friendly mouse or a sleepy cat that could also be important characters in the story.
The main characters would surely be the birds in the barn. Maybe there is a little sparrow that is very curious and adventurous, always exploring the corners of the barn. There could also be an owl that lives in the rafters of the barn and is seen as a bit of an enigma to the other birds. And if there are other animals like a dog that sometimes comes into the barn, it could also be part of the story's character set, but the birds would be at the center of it all.