1 66 



THE BIOLOGY OF BIRDS 



insecure assumptions it has been calculated that Hooded 

 Crows crossed the North Sea at a rate over loo miles an 

 hour ; that the Arctic Blue-Throat, leaving Africa in the 

 evening reaches Helgoland next morning, having flown 

 1600 miles in nine hours, i.e. at a rate of over 170 miles an 

 hour ; and that the Virginian Plover covers 3200 miles in 

 fifteen hours, i.e. at a rate of over 200 miles an hour. This 

 is preposterous. 



A critical discussion by Lucanus (1921) disposes of such 

 exaggerations. A stork does a good day's work when it 

 covers 125 miles, though it may sometimes fly twice as far. 

 It does not usually fly for more than six hours in a day. 

 A quickly flying snipe, with the wind in its favour may 

 cover 400-500 kilometres in 5-6 hours, i.e. at the rate of 

 about 50 miles an hour. 



A few figures of average migration-rate may be 

 cited : — 



The averages were established on the observed time 

 taken to fly a measured distance (about a third of a mile) 

 with the wind favourable and unfavourable. With a 

 following wind the speed is increased by a considerable 

 fraction, with a head wind decreased ; but there are limits, 

 of course, to this. When the wind attains a high velocity, 

 migration comes to an end. 



{d) Order of Coming and Going. — The general pro- 

 position is probably true that the first arrivals in spring in a 

 North Temperate country are the mature males ; they are 

 followed a little later by the mature females, who some- 

 times find a nesting-place chosen for them ; the immature 

 birds arrive last. More precise data are required before 



