To the north of the vast belt of coniferous forest that circles the northern hemisphere in the bleaker sections of the Palaearctic and Nearctic continents, the trees give way to a cold region of open landscapes called the tundra. Here the conditions are too tough even for the hardiest of trees, and the vegetation consists of short grasses, mosses and lichens. During the long northern winter, when the sun does not rise for weeks or months at a time, nothing grows. Then, when the bleary summer finally dawns, the winter snows melt away and all the plants grow frantically to best harness the short living season. Animals migrate northwards from the coniferous forests where they have wintered, to take advantage of the sudden harvest of food. They spend the summer months moving northwards, grazing as they go.
The largest animal to migrate in this fashion is the tromble, a massive flightless bird that evolved when the tundra regions appeared during the Ice Age. There are no dinosaurs this far north. Even though the dinosaurs developed a warm-blooded system that enabled them to survive a great range of climates, the chill tundra environment was still too extreme for them. The warm-blooded physiology of birds was much more efficient and therefore the birds were able to move into this harsh ecosystem. Several ground-dwelling birds evolved here since there were no terrestrial predators to threaten them.

The tromble, a 3 metre (10 ft) high flightless bird, with legs as massive as tree-trunks, migrates in huge herds across the waterlogged landscape of the summer tundra above. Eggs are laid in temporary nests at the northernmost point of the migration, and they hatch very quickly with the young able to travel immediately.

The very large body of the tromble helps to retain its natural warmth. A large animal such as this has a small surface area compared to its bulk, and heat cannot escape easily. The thick coat of hair-like feathers helps the insulation, especially during the winters spent in the shelter of the coniferous forests to the south.