172 THE BIOLOGY OF BIRDS 



belief that the young birds often migrate alone. At the 

 end of the northern summer the young birds seem often to 

 leave before their elders, and the exceptional case of the 

 cuckoo, where the birds of the year migrate last, is not 

 easier to explain. 



(d) There remains the theory that birds have in a high 

 degree what many animals show in a less degree — a sense 

 of direction, the physiological basis remaining unknown. 

 It is well-known that homing-pigeons are able to return to 

 their cots from great distances, and this has been utilised by 

 man for two thousand years. It can be improved in the 

 individual by training, but the natural gift remains rather 

 mysterious. A capacity for finding the way home is also 

 exhibited by cats and dogs, horses and cattle, but the data 

 are as yet too anecdotal to be of much value for scientific 

 purposes. 



Careful and prolonged experiments with ants and 

 bees, which find their way home from a limited distance, 

 favour the conclusion that the individual animals learn 

 their area. They build up a knowledge of their environ- 

 ment, enregistering the significance of various sets of 

 stimuli — olfactory, tactile, visual, and kinaesthetic (memory 

 of movements taken). It must be admitted, however, that 

 while we may go far with this theory of enregistering sensory 

 stimuli so as to find " landmarks " in scents, surface, 

 illumination, and so on, and of the ant's remembering the 

 turns and twists of its jungle-path, and of the bee's remember- 

 ing the general direction of its flight from the hive towards 

 which in returning it makes its " bee-line," there are 

 residual phenomena as difficult to account for as the homing 

 of pigeons. 



Professor J. B. Watson and Dr. K. S. Lashley have 

 made important experiments with the Noddy Terns and the 

 Sooty Terns which breed in tens of thousands upon Bird 

 Key, one of the Tortugas group. It marks the northern 

 limit of the migration of these two tropical terns, so that if 

 the birds are taken anywhere to the north they will find 

 themselves in all probability in a region which they never 



