From the Book of Jubilees. The Ethiopian Bible. Another apocrypha source.

B: THE ANGELS OF GOD and The Giants in Jubilees

We read further in The book of Jubilees:

Chapter 5 1 And it came to pass when the children of men began to multiply on the face of the earth and daughters were born unto them, that the angels of God saw them on a certain year of this jubilee, that they were beautiful to look upon; and they took themselves wives of all whom they chose, and they bare unto them sons and they were Giants.

The same story as in the Bible.

Chapter 5. 2 And lawlessness increased on the earth and all flesh corrupted its way, alike men and cattle and beasts and birds and everything that walks on the earth all of them corrupted their ways and their orders, and they began to devour each other, and lawlessness increased on the earth and every imagination of the thoughts of all men (was) thus evil continually. 3 And God looked upon the earth, and behold it was corrupt, and all flesh had corrupted its orders, and all that were upon the earth had wrought all manner of evil before His eyes.

You can imagine that when the offspring of the daughters of men and the Angels of God were Giants, then their 'elders' must have been Giants too.

We read further in The book of Jubilees:

Chapter 7. 21 from fornication and uncleanness and all iniquity. For owing to these three things came the flood upon the earth, namely, owing to the fornication wherein the Watchers against the law of their ordinances went a whoring after the daughters of men, and took themselves wives of all which they chose: and they made the beginning of uncleanness. 22 And they begat sons the Naphidim, and they were all unlike, and they devoured one another: and the Giants slew the Naphil, and the Naphil slew the Eljo, and the Eljo mankind, and one man another. 23 And every one sold himself to work iniquity and to shed much blood, and the earth was filled with iniquity. 24 And after this they sinned against the beasts and birds, and all that moves and walks on the earth: and much blood was shed on the earth, and every imagination and desire of men imagined vanity and evil continually.

Noah told his children, after the flood, why the Flood had been brought to the Earth. Here we can read also about the Nephilim (Naphidim) even as in the Bible, even we found a new name for the Giants, namely the Eljo. Even the Giants (Naphidim, Naphil, Eljo) sinned against the beasts and birds, You can imagine that the enormous number of stories in Myths and Legends about half Men-half Beast and half Men-half bird could be true, and it was true, they lived indeed a long ago and even until historical times (see later the story of Demons and Monsters). [link to earth-history.com]