according to their birth
Exodus 1:1-4
1
And these [are] the names of the sons of Israel who are coming into Egypt with Jacob; a man and his household have they come;
2
Reuben, Simeon, Levi, and Judah,
3
Issachar, Zebulun, and Benjamin,
4
Dan, and Naphtali, Gad, and Asher.
Genesis 43:33
And they sit before him, the first-born according to his birthright, and the young one according to his youth, and the men wonder one at another;