Many Species of Nature from Whales to Butterflies have learned that the Environment changes and that they need to adapt or move out if they wish to survive when conditions become hard. Birds are no different. The key to life is food, and if the Environment changes and the flowers dont open, the seeds stop growing or the insects move elsewhere, then you have to make a plan. Migration is a Key way of adapting to the change of the Environment that become too harsh, and many areas that are very far north (and experience climate extremes) also hold a large percentage of Bird Species that Migrate. In the USA alone 520 out of 650 species Migrate.

Some Species however have managed to adapt to the changes. Common Poorwill (a type of Nightjar) have learned to Hibernate for the Winter like many mammals do. Others like the Spectacled Eider live happily with plenty of food in the tundra areas of Canada in Summer. But in Winter when the Arctic freezes, the entire World population congregates in 6 isolated places in tightly packed groups. Their movement and heat generated at these places prevents the ice from freezing in winter, creating a hole in the ice which they can use to continue fishing from, thus surviving the Winter without Migrating.

So Why do they do it?
To live a comfortable life, Birds need Food, Water, Protective Cover and a Sheltered Nest. Unfortunately, Winter brings new conditions such as Trees stop Fruiting, most streams freeze over, most Trees lose their Protective Cover of Leaves and the Cold Weather is not favourable to raise offspring. Because great competition would result for the last remaining resourses, it becomes best to adapt to the change.