Isidore of Seville [7th century CE] (Etymologies, Book 12, 7:74): The goldfinch [carduelus], because it is nourished by thorns and thistles [carduus], whence among the Greeks it is also called acalanthis, that is, “thorn”, by which it is nourished. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.39): The carduelis is a small bird, having its name from this, as Isidore says, that it feeds on the carduis [thistles, thorns]; and it seems strange that this bird is nourished by the sharp thorns of thistles. This bird is attractive with black and crimson on the body and red color on the head. It is commonly said about this bird that when it is closed in a cage, from below by means of a thread it pulls a suspended vessel towards itself with its beak or its feet, and when it reaches the vessel it quenches thirst with a drink; and this indeed is a miracle of nature, which has given to a little fowl such a shrewdness, which neither an ox nor an ass or any large animal has been given. - [Badke translation/paraphrase]