CHAPTER 6 



NAVIGATION BY SEA-BIRDS 



WE HAVE SEEN sca-birds, especially the tubenoses^ in the Atlantic 

 many hundreds, often well over a thousand miles, from the nearest 

 continent, and sometimes several thousand miles from their breeding- 

 grounds. As the open sea to a man without instruments of navigation 

 is a seemingly trackless desert, we marv^el at the ability of the sea-bird 

 to fly home to a remote island or cliff many weeks' journey from its 

 wintering grounds. Thus the arctic tern crosses almost from pole to 

 pole, from the Antarctic to Greenland. The great and sooty shear- 

 waters breed on islands in the extreme South Atlantic bordering the 

 Antarctic, but winter in the North Atlantic (thus enjoying perpetual 

 summer) as far as the east coast of North America, Greenland, Iceland 

 and the west coast of Europe. Wilson's petrel, breeding along the edge 

 of the Antarctic Continent, performs one of the longest migrations 

 known, one of about 7,000 miles each way. During most of this journey 

 from the south polar sea to the Newfoundland Banks or to the Bay 

 of Biscay or the seas between, it may never see the land, yet this small 

 fragile-looking bird is able to return to the same nesting hole and mate 

 at approximately the same date each year of its adult life. It may be 

 argued that if W^ilson's petrel continued to fly south it would be bound 

 to strike the Antarctic Continent and so have a landmark to guide it to 

 its breeding island or shore in the nearby seas. But how does the great 

 shearwater, breeding exclusively on the Tristan da Cunha islands, 

 and migrating six thousand miles northwards in the southern winter, 

 find its home islands, which are 1,500 miles from the nearest land, 

 whence, as Wynne-Edwards points out, they subtend an arc of only 

 15 minutes. It is possible that Tristan normally provides a bigger 

 target than that, for it projects some thousands of feet into the sky 

 and for most of the time produces a cloud-mass which is high and 

 spreads for many miles. And it is possible that great oceanic islands, 

 such as Tristan, even if their actual area is fairly small, may produce 



