Isidore of Seville [7th century CE] (Etymologies, Book 12, 7:13): The bird called gradipes by the Greeks is our ‘bustard’ [tarda, “slow”] because, held back by its heavy flight, it cannot mount high on the speed of its wings like other birds. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Bird 5.21): Bistarda is a bird that takes its name from its nature. It is a bird about the size of an eagle, which in its slow flight makes two or three leaps on the ground before it is lifted up to the air, whence it is said to have its name bistarda [tarda, slow]. This bird has a hooked beak and talons like an eagle. It feeds on meat, but by no means captures prey in flight. But if it finds a sheep or any other unoffending animal, a number of the birds together fly down and kill it, and are thus satisfied with its flesh. They also eat carrion very willingly. This is almost the only bird among those that feed on flesh, that grazes on grass, and this very willingly eats peas when it is on the grass. In the wings and in the tail it has a white color, the rest of the color imitates the eagle. It lays its eggs on the ground and there it nurtures them while the crop is in fruit; and this is the reason why it is not in trees: because it cannot fly high - [Badke translation/paraphrase]