Scrolling is the migration. The view starts at the Canadian border and moves south. As you pass over a state, its students' butterflies take off and join the crowd, which grows all the way to the fir forests of Michoacán.
Every butterfly starts somewhere random inside its state, never at a city or school.
Nationwide sample: only Georgia and Oklahoma are real so far. The other states reuse the same 93 butterflies to show how the crowd builds.
Length is adjustable. The journey is about 3,000 km; the scroll length and speed can be set to match.