The halcyon is a sea bird, which lays its eggs along the shore, or deposits them in the sand.  And it lays in the middle of winter, when the violence of the winds dashes the sea against the land.  Yet all winds are hushed, and the wave of the sea grows calm, during the seven days that the halcyon sits. “Fœtificant bruma, qui dies halcyonides vocantur, placido mari per eos et navigabili, Siculo maxime.  Plin. x. 47.  cf. Arist., H.A. v. 8, 9, and Ælian, H. N. i. 36.  So Theoc. vii. 57: Χ᾽ ἁλκυόνες στορεσεῦντι τὰ κύματα, τάν τε θάλασσαν Τόν τε νότον τόντ᾽ εὖρον ὃς ἔσχατα φυκία κινεῖ Sir Thomas Browne (Vulgar Errors) denies the use of a kingfisher as a weather-gauge, but says nothing as to the “halcyon days.”  Kingfishers are rarely seen in the open sea, but haunt estuaries which are calm without any special miracle.  Possibly the halcyon was a tern or sea-swallow, which resembles a kingfisher, but they brood on land.