What about the birds?

Birds have different kinds of feathers. One kind of feathers called down. Down feathers are small and soft. They grow all over a bird’s body. Down feathers form a soft, loose layer around a bird, Heat from the bird gets trapped in the down. The trapped warm air helps the bird stay warm.

In the winter the birds migret to another hot place such as shouth asia. the birds go their for some month. & when the temperature up then they fly to their home.

More often than not, they would migrate...which means they would travel to areas that's warmer. Sometimes, this migration would take them cross country. If they can't migrate, then they are already built for that cold temperatures. Penguins are an excallent example. They have extra fat and special feathers to keep them warm. Watch "March of the Penguins" and you'll understand. :-)