WEATHER IXFEUENCES 185 



Redbreast, Wheatear, Whitethroat, Willow - Warbler, 

 Swallow, Martin, Swift, and Cuckoo. 



During August the regular departure movements 

 of the autumn set in, and are usually performed under 

 ordinary conditions — namely, fine weather. The influ- 

 ences, other than normal, are the same ungenial spells, 

 especially if accompanied by cold, alluded to for July. 

 These, however, are not frequent in most seasons, and 

 yet few seasons are entirely free from them. 



In September there are usually recorded some 

 very decided movements, which may be fairly termed 

 emigratory "rushes." These occur simultaneously with 

 the advent of weather spells which, among other 

 characters, are remarkable for a decided fall in 

 temperature, sometimes amounting to many degrees. 

 In one instance, on 15th September 1886, the difference 

 in temperature amounted to as much as 20" in twenty- 

 four hours, and naturally produced a marked effect on 

 the emigration returns. The conditions causing such 

 decided falls in the thermometer are chiefly northerly 

 winds. Sometimes, however, these cold spells pre- 

 vail with a light southerly wind, as was the case on 

 5th September 1885, when a cold, showery period 

 caused much emigration. That low temperatures are 

 the prime factors, is clearly demonstrated by the Sep- 

 tember records, inasmuch as there are unsettled periods 

 which are not characterised by cold, whose influence 

 on migration is comparatively insignificant. 



The British autumnal emigratory movements of 

 late September, October, and early November, are, 

 in their meteorological aspects, identical with those 

 which are associated with similar movements from 

 Northern Europe, except that it is essential the 



