THE GREAT WAXWING INVASION OF 1 92 1 I97 



pressure fell away to about 1020 m.b. on the 28th, 29th, 

 and 30th November, and the Waxwing numbers fell to a 

 low ebb on the 29th, with only three birds. Again a rapid 

 barometric rise culminated in 1030 m.b. on 4th December, 

 and about this time, probably a day or two before, the last 

 wave of immigration was noted. 



Allowance must, of course, be made for the time taken 

 by the Waxwings to cross from Norway to Britain, and, 

 taking this into account, it is evident that the birds did 

 not set out on migration at the barometric maxima. They 

 left, my records tell me, when, after a period of comparatively 

 low pressure, the barometer made a sudden substantial rise. 



Further, no one supposes that the Waxwings, compelled 

 to leave Norway, directed their course to Britain in the 

 hope that there hospitality would be afforded them. I 

 assume that they simply made a southward flight, and this 

 assumption, together with their appearance in Britain, 

 necessitates the occurrence at the time of migration of a 

 wind which carried them westward at the same time that 

 they were flying south. Examination of the wind directions 

 in southern Norway and over the North Sea at the times 

 of migration entirely bears out this conclusion. The dates 

 when the wind direction was most unfavourable for carrying 

 migrants from Norway to Britain were the 12th, 25th, and 

 29th November and 4th December, on each of which there 

 was a definite westerly element in the wind, and on the 

 days following these dates one, four, and five Waxwings (not 

 necessarily new arrivals) were seen. The dates when wind 

 direction was most likely to carry birds from Norway to Britain 

 were the 17th to 20th November, and the 2nd and 3rd 

 December, when an east wind was blowing in southern 

 Norway, and a south-east in the western North Sea, and on 

 the 1 8th November 127 Waxwings, and about the 2nd and 

 3rd December some 75 Waxwings were seen. 



I have said that in the absence of favourable winds 

 we need not expect to observe the arrival of Waxwings, 

 and it may be argued that the favourable wind is not only 

 a determining factor, but the prime factor. I do not think 

 that this is so ; on the 27th and 28th November the winds 



