XLVI. Songbirds apart from some exceptions do not ordinarily breed before the spring equinox or after the autumn one; and their eggs laid before midsummer are doubtful, but those after midsummer are fertile.

XLVII. Kingfishers are especially remarkable for this: the seas and those who sail them know the days when they breed. The bird itself is a little larger than a sparrow, sea-blue in colour and reddish only on the underside, blended with white feathers in the neck, with a long slender beak. There is another kind of kingfisher different in size and note; this smaller kind sings in beds of rushes. A kingfisher is very rarely seen, and only at the setting of the Pleiads and about midsummer and midwinter, when it occasionally flies round a ship and at once goes away to its retreat. They breed at midwinter, on what are called "the kingfisher days", during which the sea is calm and navigable, especially in the neighbourhood of Sicily. They make their nests a week before the shortest day, and lay a week after it. Their nests are admired for their shape, that of a ball slightly projecting with a very narrow mouth, resembling very large sponges; they cannot be cut with a knife, but break at a strong blow, like dry sea-foam; and it cannot be discovered of what they are constructed: people think they are made out of the spines of fishes prickles, for the birds live on fish. They also go up rivers. They lay five eggs at a time.

Pliny the Elder "Natural History", X.46-47.88-91 (112: Vol.III, p.349-351)

25. The alcyon (i.e. "halcyon", in classical Latin, "kingfisher"), a sea bird, is named as if the word were ales oceana (ocean bird), because in winter it makes its nest and raises its young on still waters in the Ocean. It is said that when they are brooding on its expansive surface, the sea grows calm with the winds silent in continuous tranquillity for seven days, and nature herself cooperates in the rearing of their young.

Isidore of Seville "Etymologies" (1175: XII.7.25, p.265)