Sources : Fetix

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.48): The fetix is also a bird, as Aristotle says, that even though it is small in body, it is nevertheless magnificent in its offspring. For this bird, as Aristotle testifies in his book De animalibus, lays twice a year and its young reach their due life and form. And this is because it waits for the due time in the summer and does not anticipate the time in advance, like a fatatore, and therefore is not frustrated in the role of the chicks worthy of the eggs. - [Badke translation/paraphrase]