What is a bird? The answer is simple – a feathered animal!
Feathers are found in all birds, yet no other animals have them. The discovery of fossils of small feathered dinosaurs revealed that birds are the only surviving descendants of those animals that dominated life on Earth for over 100 million years.

Snow Bunting by Bill Thompson Public Domain via Flickr

Archaeopteryx fossil by James L. Amos CC0 1.0 Universal Public Domain Dedication
via Wikimedia
Feathers probably evolved as a covering to keep birds warm, because like the mammals (including humans), birds are ‘warm-blooded’.
But most birds also need to fly, so their covering must be light-weight.
Indeed, feathers are the lightest natural material for their size.
The wings of birds, unlike those of bats and insects, are formed by feathers, which overlap and stop air from leaking through when a bird flaps during flight.
The feathers of penguins are very short as they cannot fly, and baby birds have soft ‘down’ which are feathers that lack a shaft or quill.
Text © Richard Noske 2019 CC BY-NC-SA

Thrush: Upper Wing by L. Shyamal CC BY-SA 3.0 via Wikimedia

King Penguin by Bernard Spragg Public Domain via Flickr