How birds arose from reptiles had been found out with discovery of fossil archaeopteryx. prior to archaeopteryx ,feathers had already evolved in dromasaurs for protecting them from cold.
In archaeopteryx feathers attached to forelimbs were used for gliding & flying. it lived about 180 million years ago.it has a size of crow ,along feathered tail,moderate sized feathered wings with three free clawed fingers,an elongated beak with teeth on both jaws & stream lined body covered with feathers. bones were solid like reptiles. because of its resemblance with lizards it is called lizard bird.