We're leaving something important out of the discussion of "Tracking Song," something that Alice Turner noted a long time ago:
http://www.urth.net/urth/archives/v0007/0595.shtml
In light of the Native American "totem story" character of "Tracking Song," and in light of the tall, winged being steering the Great Sleigh, and in light of the fact that the people of the Great Sleigh are bringing the sun to thaw out the frozen waste, that adds yet another layer to "Tracking Song": Has anyone ever heard of the Native American story "Raven Steals the Sun"? So there's the answer to who's in charge of the Great Sleigh: it's Great Raven. Great Raven is a trickster god and a creator god, and the bringer of daylight, so it's easy to see why Wolfe would be interested in him. More information about this legend will no doubt provide more clues to interpreting "Tracking Song." This is a truly fun Wolfe story -- so many layers! This guy should be in, like, some kind of hall of fame or something.
Matt G.
P.S.: By the by, in a quick look online, some legends of Raven say that he is a shape-shifter. Could it be that the three other giants, Nashhwonk, Mimmunka, and Ketin, are all really Raven?