"Behind me, towering almost 100 feet into the air, was a perfect ziggurat, the Temple of Kukulkan. Its four stairways had 91 steps each. Taken together with the top platform, which counted as a further step, the total was 365. This gave the number of complete days in a solar year. In addition, the geometric design and orientation of the ancient structure had been calibrated with Swiss-watch precision to achieve an objective as dramatic as it was esoteric: on the spring and autumn equinoxes, regular as clockwork, triangular patterns of light and shadow combined to create the illusion of a giant serpent undulating on the northern staircase." (Hancock)

Like the path of the sun on the frieze on the Gateway of the Sun in Tiahuanaco, meandering up and down, the Feathered Snake (Kukulkan, Quetzalcoatl etc) is the path of the sun.

(Posnansky)

Feathers because the sun is flying high in the sky - it surely must be a bird. But its path undulates (as observed against the landmarks on the horizon) and therefore it must also be a snake.