1919-] Hei(/ht at li^hkh Birds migrate. 323 



The birds most frequently observed appear to have been 

 Green Plover or Lapwings (VanellHS vanellus), and I have 

 fourteen records of this species between 2000 ft. and 8500 ft,, 

 the majority being about 5000 ft. or 6000 ft. They were met 

 with in flocks during the spring and autumn passage, the 

 earhest dates being 1 February, 1918, and 15 July, 1917. On 

 26 February, 1917, Col. C. F. Portal, D.S.O., M.C., cf 

 No. 16 Squadron, encountered a flock of Green Plover at 

 6000 ft. over Candas. " These veere flying at an air-speed of 

 about 50 m.p.h. As they were heading more or less north, 

 from Avhich direction a very strong wind was blowing, their' 

 progress was almost negligible. At lower elevations the wind 

 was more favourable, and had they chosen to fly close to the 

 ground it would have been very nearly behind them. One 

 wonders why they chose such an unfavourable current ! '^ * 



Geese and Duck have also been encountered on a number 

 of occasions, and I have records of seven instances. Col. 

 Portal met with birds of this family at a very considerable 

 height, and has published the following note: — "While 

 flyint; on duty between Bethune and La Bassee at a height of 

 8500 ft. this afternoon (26 November, 1915), I was astonished 

 to see a flock of 500 Ducks or Geese passing over Bethune 

 at least 3000 ft. above the level of our machine. The wind 

 was about 45-50 m.p.h. N.N.E., and the birds were travelling- 

 due south," 



The late Major MacCudden, V.C., informed me that he 



* Normally the velocity of the wind increases rapidly as one rises 

 above the earth's surface, and it is fairlj^ safe to assume tliat its strength 

 will be at least doubled within the first 1500 ft. At greater elevations 

 the rate of increase is usually not so rapid. An east wind generally 

 attains its maximum strength at 3000 ft., but winds from other directions 

 may increase up to 30,000 ft. 



The direction of the wind also changes very considerably as one 

 ascends. It almost invariably veers, that is to say, alters in a clockwise 

 direction, as one rises. It is quite usual for the wind to veer 40° or 50° 

 in the first few thou.sand feet, and with an east wind (which is often 

 comparatively shallow; there is frequenth' a complete reversal of 

 direction, the flow of upper air being from the west instead of the east. 



These important facts are generally overlooked by writers discussing 

 the effect of wind upon bird migration. 



