Sources : Sea-swallow

Pliny the Elder [1st century CE] (Natural History, Book 9, 43): The swallow-fish [hirundo] flies just exactly like a bird... - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.43): The sea-swallow (irundo maris) is a fish of the sea, as Pliny says, very similar indeed to the swallow bird. This fish and another fish of the sea has a nature contrary to all other fishes: for it remains as a fish in the water, and having wings is raised to the layers of the air. These fishes most aptly symbolize those who, although they are sometimes involved in worldly affairs out of duty, yet at the time and place when they have returned to themselves and are mindful of heavenly sweetness, they are more eagerly raised to the contemplation of spiritual things. - [Badke translation/paraphrase]