Migrating birds don't need to be told twice to get going.
The snow and cold looming in late October pushed straggling migrants on their way, even without benefit of a weather service. Most of them left weeks ago, some of them months before that.
Migration depends on sufficient fuel as the journeys begin. One gram of fat will fuel about 125 miles of flight for one of the warblers that nest here, according to research.
A magnolia warbler, for example, might winter near San Jose in Costa Rica. It's a flight of 3,800 miles from northeastern Minnesota, where it nests.
That warbler weighs about 8 grams, three-tenths of an ounce — feathers, bones, beak and all. Migrants can fatten for the trip, some of them doubling their weight before takeoff. That new weight will be fuel.
Add that to the bird's usual weight, and a full tank will carry the bird about 1,400 miles, give or take. You see the problem.
This bird must stop to eat, to refuel, more than once before the journey from Minnesota ends.
The route map is imprinted in the warbler's brain. Refueling points are not chosen as migration progresses. The bird is relying on a genetic map.