A Poem by Agathias Scholasticus

Greek Anthology 10.14 (Agathias Scholasticus), translated by Guy Davenport in Thasos and Ohio: Poems and Translations 1950-1980 (San Francisco: North Point Press, 1986), p. 56, with the title Priapos:

The ocean is calm, dark as wine.
No wind to edge the waves with white
or comb them with a wrinkling line,
spray them up rocks, sink them from sight.

Two swallows bright in springtime air
fetch straws to plait into a nest.
Take heart, O sailor, from this pair.
Sail to the Syrtis in the west

And to Sikilia. The price
to pay Priapos, if you wish
to get there, is to sacrifice
One gurnard, one red parrot fish.

A more literal translation by W.R. Paton:

The deep lies becalmed and blue; for no gale whitens the waves, ruffling them to a ripple, and no longer do the seas break round the rocks, retiring again to be absorbed in the depth. The Zephyrs blow and the swallow twitters round the straw-glued chamber she has built. Take courage, thou sailor of experience, whether thou journeyest to the Syrtis or to the beach of Sicily. Only by the altar of Priapus of the harbor burn a scarus or ruddy gurnards.

The Greek:

Εὔδια μὲν πόντος πορφύρεται· οὐ γὰρ ἀήτης
  κύματα λευκαίνει φρικὶ χαρασσόμενα·
οὐκέτι δὲ σπιλάδεσσι περικλασθεῖσα θάλασσα
  ἔμπαλιν ἀντωπὸς πρὸς βάθος εἰσάγεται.
οἱ ζέφυροι πνείουσιν, ἐπιτρύζει δὲ χελιδὼν
  κάρφεσι κολλητὸν πηξαμένη θάλαμον.
θάρσει, ναυτιλίης ἐμπείραμε, κἂν παρὰ Σύρτιν,
  κἂν παρὰ Σικελικὴν ποντοπορῇς κροκάλην·
μοῦνον ἐνορμίταο παραὶ βωμοῖσι Πριήπου
  ἢ σκάρον ἢ βῶκας φλέξον ἐρευθομένους.

Davenport insists on a couple of swallows ("two," "pair"), while Agathias Scholasticus mentions only one (χελιδών). Perhaps Davenport wanted to emphasize the fact that, among some species of swallow, males and females cooperate in nest building.

The sacrifice of fish was rare in ancient Greece.