Sources : Zelencides

Pliny the Elder [1st century CE] (Natural History, Book 10, 34): There is a species called birds of Seleucis for whose arrival prayers are offered to Jupiter by the migrant, inhabitants of Mount Cadmus when locusts destroy their crops; it is not known where they come from, nor where they go to when they depart, and they are never seen except when their protection is needed. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.121): Zelencides are birds, as Pliny says, which the inhabitants of Mount Cadim ask God to send when their crops are ravaged by locusts. They therefore come, as is most certainly believed, by the order of God, and destroy the locusts. And yet from whence they come, and whither they go, it is not ascertained, as they are not seen except when the people of that land are in want of their help. - [Badke translation/paraphrase]