A bird the weight of a slice of bread, flying to South America nonstop.

The engine behind the migration is Corophium volutator — a tiny mud shrimp that lives in the Fundy flats in enormous numbers.

As the tide exposes the mudflats, sandpipers feed intensively on mud shrimp and other small invertebrates. Over roughly three weeks, they can double their body weight, building the fat reserves needed for their long flight south.

When the tide rises and covers the feeding flats, the birds move to coastal roosts to rest and conserve energy.

As the rising tide covers the mudflats, thousands of birds gather on beaches and other safe roosting sites. Then, sometimes suddenly, an entire flock lifts together — flashing dark and white as hundreds or thousands of birds turn in perfect unison.

The tight flock provides protection from predators such as peregrine falcons while the birds rest between feeding periods.

But the spectacle comes with a cost: every unnecessary disturbance forces the birds into the air and burns precious energy they need for the Atlantic crossing.

Semipalmated means "half-webbed." The species has short webs between some of its toes, an adaptation that helps it move across soft mud.

It is a tiny feature on a bird making an enormous journey.