Sources : Trochilus

Pliny the Elder [1st century CE] (Natural History, Book 9, 37.90): [The crocodile] when sated with a meal of fish and sunk in sleep on the shore with its mouth always full of food, is tempted by a small bird (called there the trochilus, but in Italy the king-bird) to open its mouth wide to enable the bird to feed; and first it hops in and cleans out the mouth, and then the teeth and inner throat also, which yawns open as wide as possible for the pleasure of this scratching. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.40): Crochilus, as Pliny says, is a bird which in Italy we call the king of birds. The bird is indeed the smallest of all birds; but, as Pliny says, the smaller the body, the greater the speed. But it is so brave that it dares to try against the eagle, the king of the birds. It is fertile with chicks, and brings up many with one brood. It feeds on spiders. It deceives men, so that when it offers itself close as if to be taken by the hand, it escapes the effort and labor of man. In winter, many are crowded into one cave, so that the warmth, which is small in the smallest bodies, may be increased, as the crowd contributes to the warmth. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.33): when he [crocodile] is full, he lyeth by the brinke or by the cliffe, and bloweth for fulnesse, and then there commeth a little bird, which is called Cuschillus among them, and is called king of foules among the Italians, and this bird flyeth before his mouth, and sometime he putteth the bird off, and at the last, he openeth his mouth to the birde, and suffereth him to enter. And this bird claweth him first with clawes softly, and maketh him have a manner lyking in clawing, and falleth anone asléepe, and when this bird Cuschillos knoweth and perceiveth that this Beast sléepeth, anone hée descendeth into his wombe, and foorthwith sticketh him as it wer with a dart, and biteth him full grievouslye and full sore. - [Batman]