The Great North Migration, Again

Every year on Planet Earth,  about this time,  a great many birds head north.  They migrate south to north. Chances are, close to a dozen could fly right over an average person's noggin, unseen, unheard, unsmelt.    Used to be, many more could fly over your noggin.  But the excess, beyond a dozen, have long since been eaten or run into some obstacle, like a wind turbine.

Crumby dwells along one of those many areas where he is assured that east is divided from west.  So Crumby may count upon six eastern birds, plus six western birds, flying over his noggin, maybe.  Some may dwell in these parts all winter.  Some may come into these parts for the summer.  Some may  pass through.  Therefore, for example, a bird that passes through is liable to be only two of the twelve, one western and one eastern.  Goodness!

Crumby is not sure how to enumerate the ones that straddle the ecotone.