Sources : Gosturdus

Thomas of Cantimpré [circa 1200-1272 CE (Liber de natura rerum, Birds 5.54)]: The gosturdus is a very small bird, very similar in color to the earth; on their heads they have a small crest of feathers. They fly, indeed, not in the manner of other birds, but imitate the undulating waves, which, driven by the wind, now go high, now low. It is generally said of these that their eggs are laid on the ground to be hatched by a toad. When the chicks are hatched, their parents take care of them for the first time and feed them until they can try to fly. But they signify some very wicked men, who were placed in our age, who, when they see their children adorned with good character, and not emulating their parent's malice, immediately regard them as strangers; and afterwards, if they turn to malice, they recognize them as their own, and administer to their wickedness the foment of their crimes. - [Badke translation/paraphrase]