To the north of the vast belt of coniferous forest that circles the Northern Hemisphere in the bleaker sections of the Eurasian and North American 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 Last Ice Age. There are no non-avian dinosaurs this far north. Even though the non-avian 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.