19 1 7- Moffat — Some Migrant Notes. 133 



have synchronized with any remarkable deviation from 

 the normal as regards weather or temperature, so that this 

 strange breach of the rule of averages remains so far in 

 want of even an hypothetical explanation. 



But of course it should be remembered that the explan- 

 ation may sometimes be sought in conditions prevailing 

 elsewhere than in our own country, and sometimes, as in 

 the case of the great bird-rush of March 29th, 191 1, so well 

 explained by Mr. Barrington in this Journal (vol. xx., pp. 

 97-110), in a complex combination of circumstances necessi- 

 tating some knowledge of atmospheric conditions extending 

 over a wide area. This is a further reason why the Royal 

 Meteorological Society should not neglect the subject. 



NOTE. 



By the average date of first arrival I mean the sum of 

 the dates divided by their number — not the mean between 

 the earhest and latest. The mean, I see, is used by Mr. 

 Foster in his note in the July number (p. 122) ; but Mr. 

 Foster himself very fairly points out that such a course 

 " may give a misleading impression as to the usual date " — 

 the reason for this being that the mean is sometimes 

 violently affected by one extreme variation. For instance, 

 the late date of the Chifichaff' s arrival at Ballyhyland this 

 year would alter the m^ean date for that migrant by as 

 much as three days, though it does not affect the average 

 date by more than half a day. In adding up the dates 

 for a bird whose arrival does not always take place in the 

 same month one must, or course, adopt a consecutive 

 numbering, as if the two months were one. Thus, in the 

 case cf the Chiffchaff, April i may be counted as March 32, 

 April 12 as March 43, and so on. 



Ballyhyland, Co. Wexford. 



