l6o SEA-BIRDS 



mechanism which is quite independent of topographical '^knowledge' 

 or memory; and the fact that the mechanism breaks down in 

 cloudy weather suggests very strongly that orientation is by the sun. 

 Kramer (1952) found that the highly accurate orientation faculty 

 in his caged starlings vanished if the sun was hidden: when tests were 

 made with an artificial sun (lamp) indoors, they orientated as if the 

 lamp were the sun. 



The sun-navigation theory, while it disposes of earlier hypotheses 

 of kinaesthetic memory and sensitivity to the earth's magnetic field 

 or Corioli's force (which depends on the velocity of movement of 

 points on the. earth's surface relative to its axis and which therefore 

 varies with latitude), does not explain night-migration, which is a 

 well-known phenomenon, although not commonly reported of sea-birds. 

 Most sea-birds in fact rest at night, either on land or sea. The activity 

 of fulmars, watched by us from a trawler, showed considerable diminu- 

 ution at night. Manx and other shearwaters assemble in rafts on the 

 sea in sight of their breeding-grounds before sunset, although they 

 often do not land until two hours after sunset. In heavy overcast 

 conditions many night-migrants appear confused and may descend 

 in their greatest numbers on the coastal bird observatories, there to 

 rest — and this is sometimes erroneously referred to as a "good migra- 

 tion." But it is on clear moonlit nights that the best migration occurs — 

 at least for the migrants, which can navigate onwards without a halt, 

 and without striking the lanterns of lighthouses, because they see the 

 outlines of the land and the astronomical signs clearly. 



Sun or astronomical navigation seems to be the most satisfactory 

 explanation so far; and more research into night-migration may 

 reveal that the migrant is guided by moon, stars, pre-glow and after- 

 glow which may assist it to keep on a course already begun before the 

 sun or its glow has left the sky. It should be remembered that night- 

 migrants only fly for a comparatively short period, for not more than 

 half the night — unless they are caught, being land-birds, over the 

 sea, when they must continue flying to the nearest land, often some 

 cape or island where there may be a migration observation station. 

 Research may also show that the sea-bird is more lost and helpless 

 in misty weather than was formerly believed; and that stories of 

 homing through dense fog are largely fictitious. Sea-birds are in fact 

 confused in heavy mist, and are then easily caught at the nest by the 

 use of a powerful torch; on such nights shearwaters will accidentally 

 strike objects over the breeding ground during flight, including light- 



