172 THE MIGRATION OF BRITISH BIRDS 



Redstart {Riiticilla pJianicunis) is known to have spread 

 northwards in Scotland of late years, and now breeds 

 sparingly to the very northern limits of the mainland. 

 To Ireland, however, as we have already seen, it is 

 purely an abnormal migrant, which shows how strong a 

 barrier a wide water area is to a successful extension of 

 range. The same remarks apply to the Shetlands ; we 

 have many species whose range extends to the Orkneys, 

 but there is little prospect of that range ever normally 

 reaching the more remote group of islets. 



In the same way the Robin (^EritJiacus riibccula) has 

 steadily extended its northern range in Scotland, and 

 now breeds regularly in the Hebrides and the Orkneys, 

 where formerly it was excessively rare or entirely 

 unknown. There is some evidence to suggest that the 

 Nightingale {Erithaais liisciiiia) is extending its range 

 northwards and westwards ; and this species very 

 curiously illustrates the potency of our Law of Dis- 

 persal, for there is net a scrap of evidence to suggest 

 that its line of Emigration is spreading south of Exeter 

 — a fact of very great significance, as we shall learn 

 in a later chapter (c-(?;//. pp. 215-217). This species is 

 extending its range higher in our area than elsewhere, 

 due perhaps to the absence of the competing form L. 

 philomela. The Whitethroat {Sylvia cinerca) appears to 

 be increasing its range in the north of Scotland and 

 becoming more dominant. The Willow Wren {P/ijIlo- 

 scopKs trocliilus) is undoubtedly increasing its area, and 

 also becoming more dominant in the north of Scotland ; 

 whilst the Wood Wren {Pliylloscopus sibilatrix) is a yet 

 more interesting instance, having spread northwards of 

 late years even to areas at the very extremity of the 



