366 Letters, Extradft, am/ Notes. [Ibis, 



and compared with types before an^^ authentic note can be 

 made on migration or any other subject ! The well-trained 

 eye of a field naturalist will detect the slightest difference in 

 birds, no matter how much alike tliey may look. Take, for 

 instance, our Common Swift and the Chinese Swifr, which 

 are found associating in enormous flocks in British East 

 Africa in November, In the cabinet they look much alike, 

 but when alive in the clear atmosphere of East Africa they 

 could not possibly l)e mistaken. Ir is when we come to 

 distinguish Ijirds by saying (often from a small scries) that 

 thev averaiic one or two millimetres longer in the wing 

 that difficulties begin, and the study of birds in the field will 

 be made impossible. AVhat we really want is a " Controller's 

 Office." Will not the B. O. U. take the raatter up and 

 see if something cannot be done to stop the confusion which 

 is bound to occur if things are left as they are ? 



Yours faithfully. 

 Govsemoor, \Yilloughby P. LowE. 



Throwleigli, Devon. 

 30 Januarv. l'Jl!>. 



Migration and Aviation. 



Dear SiK, — May I make use of ' The Ibis ' in order to 

 try to collect whatever data are available regarding obser- 

 vations on the migration of birds made in the air by the 

 Royal Air Force? Perhaps any members of the B. 0. U. 

 who are able to tap such a source would be so kind as to 

 place me in communication with those officers who have 

 made any notes on the subject. 



In response to a recent advertisement in the ' Times/ 

 I have had a number of replies, of which a cursory examina- 

 tion in the light of existing knowledge on the height at 

 which birds travel and the rate at which they fly when 

 travelling, confirms what I suspected, that birds seldom 

 travel by day at elevations much exceeding 2000 feet, and 

 that their velocity of flight, with a few exceptions, rarely 

 exceeds 50 miles per hour. But on tiie very scanty evidence 

 available, it is dangerous to theorise ; but it is in the hopes 



