86 BIRD LIFE THROUGHOUT THE YEAR 



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 re- 

 appearance 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 late- 

 comers 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 develop- 

 ment 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, 



