158 SEA-BIRDS 



minute changes in the surface and topography of the sea which may 

 be detectable many scores of miles away by birds. 



Many experiments have been made with sea-birds with the object 

 of discovering the physical nature of the mechanism of homing birds. 

 Visual memory can serve where there are familiar land or sea marks, 

 and birds experimentally released within sight of these do orientate 

 themselves and fly home very quickly. But where they successfully 

 return when released far from home and in country or upon seas which 

 they are presumed never to have visited before, visual memory as an 

 aid is out of the question. The classical experiments with noddy and 

 sooty terns carried out by Watson and Lashley (19 15) are well known. 

 Breeding adults from the Tortugas Islands, Gulf of Mexico, were sent 

 by ship for distances of more than 850 miles, and returned from points 

 far to the north of their normal distributional range. Griffin sum- 

 marised these and other homing experiments which included terns, 

 gulls, petrels and shearwaters; he concluded that some birds return 

 home by a process of exploration in search of familiar visual landmarks. 

 A gull released over the land and followed in an airplane flew at 

 random until it sighted the ocean, towards which it then headed 

 directly, although this particular part of the sea was neither the 

 nearest salt water to the point of release nor the direction in which 

 its nesting ground lay. Griffin and Hock (1948) took gannets from 

 Bonaventure in the Gulf of St. Lawrence, released them over a hundred 

 miles from the coast and followed them by air, sometimes for more 

 than a hundred miles. The paths taken by the birds consisted of fairly 

 long "legs" with sudden sharp turns, and somewhat resembled a 

 cubist's idea of a spiral search, though the most simple explanation 

 of this search is that it was quite at random. About sixty per cent, 

 of the gannets released eventually reached home at a speed of about a 

 hundred miles a day; those followed by air homed equally with a 

 group of control birds that were not followed. 



Two Manx shearwaters which returned to Skokholm after release 

 at Venice have provided an early example of a sea-bird returning 

 from an almost land-locked sea which it never normally visits (Lockley, 

 1942). A significant fact about this release was that one of the shear- 

 waters, instead of heading for the open sea (Venice is 3,700 miles by 

 sea from Skokholm, and a bird taking the sea-route would have to 

 begin by flying in a direction directly opposite to that of Skokholm) , 

 rose up into the air and flew inland westwards in the direction of the 

 sun, which was at that moment sinking behind the Italian Alps! 



