Every autumn across North America, migration begins.

And across the continent's highways and desert roads, another migration gathers - this one made not of birds or fish, but of humans.

They go by many names: nomads, drifters, snowbirds, boondockers, van dwellers. Some travel in search of warmth, others for freedom and community. And for a growing number, the migration is not simply seasonal but economic.

Among those is 55-year-old Derek Hansler, a chef by trade.

Known to friends as D Rock, he spends the summer in New Hampshire visiting his children and grandchildren, parking his 2003 Van Terra shuttle bus in driveways along the way. He picks up gigs when he needs cash or a place to park, but the season is less work than service, volunteering in the communities he revisits every year.

"New Hampshire tells me when it's time to roll," he jokes. He likes to stay until the leaves turn crimson, then leave before they fall. When that moment arrives, he says goodbye to his family and points his bus 3,300 miles (5,310km) to the south-west.

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