Migration occurs roughly every half hour. However fitness evaluation is noisy so the same individual will get a range of scores. Currently the fitness values you see are the average of 30 evaluations. However as we approach areas of higher fitness it might be better to put this up a bit to make the fitness scores more accurate. The problem is this makes the whole thing slower. So for now I'll leave it like this. As long as the whole cluster's fitness is going up.
Hope this helps,
Miguel