"Birds" represent rational concepts in the external man and intellectual concepts in the internal man.

3624. THE CORRESPONDENCE WITH THE GRAND MAN, WHICH IS HEAVEN, OF ALL THE ORGANS AND LIMBS, BOTH INTERIOR AND EXTERIOR, OF THE HUMAN BEING

Let me now tell of and describe marvels which, so far as I know, have not as yet been known to anyone, nor has even the idea of them entered anyone's head - the marvel that the whole of heaven has been formed in such a way that it corresponds to the Lord, to His Divine Human; also the marvel that the human being has been formed in such a way that every single part of him corresponds to heaven, and through heaven to the Lord. This is a great mystery which is now to be revealed, it being the subject here and at the ends of chapters that follow this.