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.