A similar article with more information about the squirrel migration of 1968: https://mentalfloss.com/article/94069/great-squirrel-migrati.... And since the official links seem to be broken, here's someone's copy of the short research paper "The 1968 Squirrel 'Migration' in the Eastern United States": http://www.myoutbox.net/flyger.htm. And to be complete, an article about the author of that paper: https://umdarchives.wordpress.com/2013/10/04/the-great-squir....

I wonder to what extent the loss of the American Chestnut affected the squirrel population.

I thought "migration" had a strictly seasonal component separate from food availability. Thank you, OP, for the challenge to my baseline!