Birds and bats fly, but they are very different animals. Indeed, their wings are the only real similarity between them. While some species of birds share some characteristics with bats (such as owls also being nocturnal like bats), the broad definitions are very different.
Feathers vs Fur
The defining characteristic of a bird is that it has feathers. Every animal with feathers -- from pigeons, to turkeys, to chickens to ducks and everything in between with feathers -- is a bird. They don't necessarily need to be able to fly (such as the emu) but they do need to have feathers. Bats, on the other hand, are mammals, all of which have fur or hair.
When a female bat gives birth, she does it the same way humans, cows and other mammals do -- her child gestates in her womb and she gives birth through her vagina. Birds, on the other hand, lay eggs. A baby bat develops inside its mother, while a baby bird develops inside an egg.
Birds have a more complex respiratory system than bats. Bats have lungs like people -- air goes in, gets processed, then goes out the same way. Since birds usually fly, their systems require more oxygen. So, a bird's respiratory system has more chambers and air sacs in order to get more oxygen out of each breath of air.
While bats do have standard vision, they are nocturnal animals. To see in the dark, they use echolocation, which is bouncing sound waves off objects and estimating how long it takes for them to get back, then using this information to paint a mental picture of the terrain around them. Birds, on the other hand, have standard vision that relies on light reflecting off surfaces. Some birds, such as hawks and owls have highly developed vision, but it is light-based vision, not sound-based.
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