Bats and birds both fly !!!
But they are built completely differently.
1. What they actually are
Bats are mammals and warm blooded.
Birds lay eggs and warm blooded.
2. Wings
In bats, wings are skin stretched.
Birds have modified forelimbs covered with feathers.
3. Bats don't have feathers. Body is covered in fur.
Birds are covered in feathers which are crucial for flight and insulation.
4. Bones
Bats : Bones are thin, flexible and lightweight. Skeleton is adapted to hanging upside down.
Birds : Bones are hollow. They have breastbone to anchor powerful flight.
5. Navigation
Bats : Rely on bouncing sound waves off objects. Eyesight is poor.
Birds : Have excellent eyesight and colour vision. They navigate using magnetic field.
6. Reproduction
Bats : Give birth to live offsprings. Mothers carry and nurse them.
Birds : .Lay hard shelled eggs and incubate them.