SOME QUESTIONS CONCERNING MIGRATION 79 



at a height of 5 miles, and have even identified them as 

 woodpeckers and so forth. Mr. F. Chapman saw no fewer 

 than 262 in five hours. Gatke was of opinion that many 

 migrating birds, such as curlews, in full flight keep at an 

 elevation of 10,000 ft. or more; while Eagle Clarke notes 

 that he saw enormous numbers of larks, starlings, and 

 thrushes flying very low across the North Sea. It is 

 probable, however, that these low-flying birds form a small 

 minority. Careful observers have recorded hearing the 

 voices of crowds of migrants passing overhead on dark 

 nights, and have argued from the distinctness of the notes 

 that the birds could not be very high. It seems unlikely 

 that birds habitually fly at a height of anything like 

 10,000 ft., for that would involve the serious disadvantages 

 of rarefied air and low temperature. On the other hand, 

 if low flying is common, we ought to see more migration 

 than we do. The fact is, we do not know much about 

 the altitude of migratory flight. 



A third question which cannot be answered without 

 more data concerns the velocity of the migratory flight, 

 in regard to which startling statements are often made. 

 It seems reasonable to allow that migration represents a 

 climax of activity, when the organism is attuned to great 

 deeds, the easy-going bird flying swiftly, the low-flying bird 

 ascending to higher strata, the diurnal bird flying by 

 night, and so on; but the estimates which even experts 

 have made of a rate of 200 miles an hour require to be 

 substantiated. 



Two or three methods have been tried, e.g., observing 

 the time birds took to pass from one visible point to another ; 

 observing the time a bird of known size took to become 

 invisible ; and arguing from the absence of records of a bird's 

 occurrence between two countries, A and B, that the bird 

 flies from the one to the other without stopping. Gatke 



