Thirdson,
In Noah's time it seems that "clean" means acceptable for sacrifice since humans did not eat animals before the flood.
Genesis 8:20 ***
20 And Noah began to build an altar to Jehovah and to take some of all the clean beasts and of all the clean flying creatures and to offer burnt offerings upon the altar.
It wasn't until the Mosaic Law that these birds were specifically named as "unclean" for eating.
Deuteronomy 14:11-20 ***
11 “Any clean bird YOU may eat. 12 But these are the ones of which YOU must not eat: the eagle and the osprey and the black vulture, 13 and the red kite and the black kite and the glede according to its kind; 14 and every raven according to its kind; 15 and the ostrich and the owl and the gull and the falcon according to its kind; 16 the little owl and the long-eared owl and the swan, 17 and the pelican and the vulture and the cormorant, 18 and the stork and the heron according to its kind, and the hoopoe and the bat. 19 And every winged swarming creature is unclean for YOU. They should not be eaten. 20 Any clean flying creature YOU may eat.
The Bible does not state specifically why the animals were unclean ceremonially. Most of the birds are birds of prey or scavengers but not all.
The vision given to Peter shows that humans are no longer bound by the Law including the dietary laws.
Acts 10:12-15 ***
12 and in it there were all sorts of four-footed creatures and creeping things of the earth and birds of heaven. 13 And a voice came to him: “Rise, Peter, slaughter and eat!” 14 But Peter said: “Not at all, Lord, because never have I eaten anything defiled and unclean.” 15 And the voice [spoke] again to him, the second time: “You stop calling defiled the things God has cleansed.”