until the return of spring. White, of Selborne, wavered, at one time inclining to the opinion that the swallows crossed the sea, at other times half expecting to hear of their being discovered in mid-winter in a somnolent condition in holes of sand-pits or under the roofs of houses. This uncertainty has, of course, long since vanished. A host of observers chronicle the reappearance of the welcome heralds of spring, so that, in addition to knowing when and upon what part of the coast they land, we may trace, step by step, their gradual dispersal over the country. Especially good work has been done by the lighthouse-keepers, some of whom have become keen observers. Captains of ships, too, take note of the feathered wayfarers which accompany their vessels at migration time, often perching exhausted upon the deck or rigging.
The whole period of migration, from the appearance of the Wheatear and Chiffchaff to that of such latecomers as the Nightjar and Red-backed Shrike, is about seven weeks. Each arrival goes hand in hand with the attainment of a certain stage in the development of leaf and herbage, with accompanying awakening of fresh insect life. The particular date for the appearance of the Wood Wren is marked by the leafing of the beeches. The aquatic warblers do not appear until the new growth of reeds affords them sufficient cover. And the Nightjar need not be expected until,
