At this point I've jumped off the migration bandwagon if, by "migration" you mean long distance seasonal travel. I do think they relocate within their range seasonally but it's a much shorter trip, probably 20-50 miles, not the hundreds I used to surmise. So .. why the change of heart? Deeper study of the report data.
What I saw was a long, somewhat thin. concentration of reports along a long mountain range. It seemed to end at a location which has a long history. Such a migration, mapped in time, should present a traveling wave of sightings ... and it didn't. Instead the activity "fired off" from one end of the range to another across well over 1000 miles. That's too far for weather change or other seasonal factors to affect all at once as well because the range runs N-S. The only thing that makes any sense is an astronomical trigger, something that appears simultaneously at all latitudes. Further study later also showed that there's a spike of activity at the south end of the area in mid summer, exactly out of phase with the presumed migration.
Today I suspect that there are "migrations" up and downhill, changes in elevation to take advantage of resources or avoid weather, but the actual distance traveled is pretty short. This makes a lot of sense because it would mean that the Cascades where I am could have the same general pattern as the Rockies where it's pretty much too far for even a bigfoot to travel to get out of the snow.
Interestingly enough, away from the mountains themselves, the number of sightings stays pretty constant year around whether it's 90 degrees or approaching 10 below suggesting extreme adaptation to variable climate ... so long as food is accessible.
For now, I'd say there are "bands", for lack of a better word, which stay in the same general area but move up or down in elevation to follow food.
It doesn't stop there. Some of the habituators I have worked with say that their local "band" is there most of the year fairly constantly but that at certain times of the year, or sometimes every second or third year, the same small group of traveling 'foots, probably sub adult and young adult males, pass through. They're often recognizable and predictable in their timing.
I note that that would be a useful behavioral adaptation for a small, spread out population to keep the genes mixing.
I don't think anyone knows for sure, though, that's just my current interpretation of behavior, timing, data, etc. I changed my mind once before, I may change it again.
MIB