In exactly the same ways as humans, short of artificial heating although they will certainly roost around warm buildings and other things in bad weather. The primary way is the fact that they are endotherms - they generate their own heat.
Feathers
All birds are warm blooded.
Birds are covered in feathers
Possibly you meant "How do the birds keep warm . . ."? As for why they keep themselves warm, it is to stay alive.
so that their baby's will develop properly , if your told its to keep them warm its wrong , ( birds sit on the eggs to keep them warm).
it is feathers
Birds Sit on their eggs to keep them warm, and to hide and protect them from predetors.
Most birds lay on their eggs to keep them warm and safe.
They keep them warm and for most of them ables them to fly.
To keep both sides warm!
How do you stay warm, friend? They invested in heaters along with the rest of us!
Keep them warm and keeps them warmkeep the birds warm and help them fly
They are avians (birds). Like mammals, all birds are warm blooded. No exceptions.