Now that this is fixed, if I understand correctly what mark said, the migrants will NOT queue and wait for the next available island to become the correct age as I suggested. BUT it will search for another island of the correct age and if it does not find the correct age the migrant is simply lost. I can add that when I was really examining the copulation function of SQX it usually produces a child as one tiny variable from one parent and 99% of the other parent (mimicking the mutation pretty much,) The copulation function does seem to work as intended but could probably be optimized.
As for filling up equally, that is weird, I reported a similar bug here but it seems to be fixed when I use 12 islands they all fill up equally fast now:
https://roadmap.strategyquant.com/tasks/sq4_7723
Status changed from New to Fixed