The flying squirrel is a mammal that can glide through the air. Here is a bird. The bird is not using power to move its wings. The bird is resting. The bird is gliding on warm air currents.
True flight is to fly with muscle power.
Insects, birds and bats use muscle power to flap their wings. These animals flap their wings up and down. Insects, birds, and bats are the only true fliers. A butterfly is an insect. A hummingbird is a type of bird.