This documentary presents an epic portrait of winter bird migration. Filmed on all seven continents over four years, the footage is brought together into one portrait of a journey that's uniformly arduous for all kinds of different birds. Rather than concentrating on statistics and facts, the film near-wordlessly portrays the sheer physical effort demanded of the birds on their disparate routes. Along the way, we see the many dangers they face, from man-made waste to a group of hungry crabs.