70 REPORT ON THE MIGRATION OF BIRDS. 



November from districts which lie directly east or south-east of 

 Great Britain. Should our English winter prove generally 

 severe, or even partially so, our old birds will also leave, and in 

 their place we have an influx of old Blackbirds from the 

 Continent, pushed forward by similar causes. In the last 

 autumn and winter, which has been exceedingly variable, with 

 many sudden changes of temperature, our East Coast reports 

 show an enormous migration of this species to have taken place. 

 This commenced on September 12th, and was continued at 

 intervals throughout October, becoming nearly continuous in 

 November ; after this intermittent, in throbs and pulsations, up 

 to the end of the third week in January, 1885. These influxes 

 of old birds correlate with sudden changes of temperature and 

 outbreaks of severe weather on the Continent. In the spring 

 the continental visitors disappear, and our so-called resident 

 Blackbirds come back to their nesting-quarters. In this way we 

 have a regular interchange of birds in the spring and autumn, — 

 a double migration, carried on twice a year between the British 

 Islands and the Continent. As far as our knowledge extends the 

 normal conditions of locality and climate over the whole area are 

 such as do not necessitate a regular interchange of the members 

 of their respective avi-faunas. There is apparently no reason 

 why our Rooks, Starlings, Sky Larks, and Blackbirds should not 

 be able to winter in England just as well as abroad ; their not 

 doing so, but persistently migrating, is suggestive of a regular 

 succession of bird waves, — as one moves forward another quickly 

 follows in its wake. The southern limit of autumn migration, 

 in the individuals of any species, is in direct ratio to the 

 breeding range ; such as nest in southern latitudes move still 

 further south, and thus make room for those of -their sort which 

 have nested further north. Such are the ordinary phenomena 

 of migration, a movement which is as regular and persistent as 

 the flow and ebb of the tide.* 



:;: Migration is a voluntary, and not a compulsory, movement; and it 

 must not be imagined that recurring bird waves actually push each other 

 forward. Young Blackbirds and Thrushes leave- some weeks in advance of 

 the great rush of their continental allies ; and it is well known that 

 Woodcocks, which breed in Great Britain, leave in the autumn, before the 

 arrival of the northern flights, a distinct interval elapsing -between the two 

 events — the going and the coming. 



