The world tree always has a bird in the branches and a serpent at the roots. Look at these two images. The serpent goddess above from the 2nd century CE, perhaps an early image of a mermaid, shows the goddess and the serpent as one. The familiar image of Isis on the right shows her with her wings. The goddess and the bird as one.
As archaeological scholar Marija Gimbutas wrote, “The Snake Goddess and the Bird Goddess appear as separate figures and as a single divinity. Their functions are so intimately related that their separate treatment is impossible. She is one and she is two, sometimes snake, sometimes bird.”[i]
AND, I would add she is all TREE OF LIFE just as the TREE OF LIFE is Her complete with Her feathered wings and Her serpent “legs.”
[i] Gimbutas, Old Europe; 112.