Pausanias [circa 110-180 CE] (Description of Greece, 8.22.5): These fly against those who come to hunt them, wounding and killing them with their beaks. All armor of bronze or iron that men wear is pierced by the birds; but if they weave a garment of thick cork, the beaks of the Stymphalian birds are caught in the cork garment, just as the wings of small birds stick in bird-lime. These birds are of the size of a crane, and are like the ibis, but their beaks are more powerful, and not crooked like that of the ibis.

Pliny the Elder [1st century CE] (Natural History, Book 11, 44): ...the bird of Stymphalus [has] a crest... - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 7:27): The stymphalis is a bird named from the islands of Stymphades, where they are abundant; Hercules used arrows against them. They are sea birds living on islands. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.117): Vanellis are birds, as he says [i.e. Liber rerum], as they are commonly called. Pliny calls these symphalides. They are quite beautiful birds of the size of a dove, with a crested head like a peacock, a neck of a green and shining color, and the rest of the body a distinct variety. This bird, when it sees a man approaching its nest even from a great distance, goes forth immediately to meet him with a cry, that is to say, expecting the man to depart because of its cry. Thus the foolish bird, when it believes that a man is chased from its nest by its cry, betrays the nest because of the cry. The the man immediately realizes that the nest of the crier is near, and with the nest betrayed, he endeavors to rob it. - [Badke translation/paraphrase]