ON BIRD MIGRATION. 405 



Some of these may not proceed at once beyond our southern counties, 

 where the length of their sojourn is determined by the climatic con- 

 ditions of the season ; others depart forthwith for more southern regions 

 beyond our limits. 



Throughout August, but chiefly towards the end of the month, there 

 are clear indications at the light-stations that Thrushes are quietly 

 slipping away from Britain. There are no marked movements recorded 

 for this month, but there is unmistakable evidence that a gradual and 

 steady emigration is in progress on all the coasts of Britain. From the 

 Irish coasts, however, this happens only rarely during August, the birds 

 usually departing later in the season. These earliest emigrants are gene- < 

 rally observed in small numbers, and either alone or occasionally in 

 company with ' "Warblers ; ' sometimes a few are killed at the lanterns. ' 



In September and October the emigratory movements are more 

 general and more pronounced in their nature ; but it is not until the 

 weather breaks up in the latter month that any ' rush ' is recorded. 

 During these months, especially in September, the Thrush departs m 

 company with various species of summer birds, and its emigrations are 

 recorded from all sections of the British and the east and southern coasts 

 of Ireland. The Thrush is, however, emigratory to a lesser degree in the 

 Sister Isle than in Britain. 



In. October the migratory movements of the Thrush are often of a very 

 complex nature, and are diflicult to interpret. The most complicated 

 movements are those during which emigration, immigration, and passage 

 are in progress simultaneously, a phenomenon which sometimes happens 

 under peculiar weather conditions.^ 



Later in the year the emigratory movements, which doubtless include 

 many of the recently arrived immigrants from the north of Europe, are 

 dependent on and synchronous with more or less severe weather con- 

 ditions, and these will be duly treated of in the proper place. 



A^itumn Immigration and Passage. — There is no evidence whatever of 

 the appearance of the Thrush upon our shores, as an immigrant from 

 North-western Europe, until the end of the third or the beginning of the 

 fourth week of September, when it arrives with great regularity "* in 

 company with the first Redwings {Turchcs iliacus) ; occasionally Red- 

 breasts {Erithacus ruhecula), Goldcrests (Eegulus cristatus), Woodcock 

 (Scolo2)ax rusticula), Jack Snipe {Gallinago gallinula), and Short-eared 

 Owls (^Asio accipitrinus) are observed at the same time.^ 



The immigrations continue during October, during which month there 

 are lulls, followed usually by two very pronounced ' rushes ' to our shores, 

 when for several successive nights Thrushes pour in upon our eastern sea- 

 board in vast numbers. These ' rushes ' occur as a rule (1) about the 

 middle of the month, and (2) again during its fourth week. 



These immigratory movements are confined to the east coast of 

 Britain, from the Orkneys to Norfolk. North Ronaldshay, the most 



' As early as August I, 1884, six Thrushes struck the lantern of Dhuheartach 

 Rock Lighthouse, two being killed. 



- See ' Digest of Observations,' Brit. Assoc. Rep., 1896, p. 471. 



^ On September 21 in 1881, 1882, 1883, and 1887. 



* Professor Collett, Oversigt af Christiania Omegns ornithologislie Fauna, p. 27, 

 says that the Thrush departs from the Christiania district during September, and 

 continues to do so until the first days of November. Statistics for S.W. Norway 

 would be preferable, as being more intimately associated with those for Great Britain, 

 but unfortunately they do not appear to be available. 



